AzevedoOLLK20153FACAzevedoMOrtiz-RiosQLiNKLogothetisGAKeliris2015-07-003-428331349Multisensory ResearchA biologically relevant event is normally the source of multiple, typically correlated, sensory inputs. To optimize perception of the outer world, our brain combines the independent sensory measurements into a coherent estimate. However, if sensory information is not readily available for every pertinent sense, the brain tries to acquire additional information via covert/overt orienting behaviors or uses internal knowledge to modulate sensory sensitivity based on prior expectations. Cross-modal functional modulation of low-level auditory areas due to visual input has been often described; however, less is known about auditory modulations of primary visual cortex. Here, based on some recent evidence, we propose that an unexpected auditory signal could trigger a reflexive overt orienting response towards its source and concomitantly increase the primary visual cortex sensitivity at the locations where the object is expected to enter the visual field. To this end, we propose that three major functionally specific pathways are employed in parallel. A stream orchestrated by the superior colliculus is responsible for the overt orienting behavior, while direct and indirect (via higher-level areas) projections from A1 to V1 respectively enhance spatiotemporal sensitivity and facilitate object detectability.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published18A Potential Role of Auditory Induced Modulations in Primary Visual Cortex1501715421ShahBKKJVV20153DShahIBlockxGAKelirisFKaraEJonckersMVerhoyeAVan der Linden2015-07-00Epub aheadBrain Structure and FunctionResting-state functional MRI (rsfMRI) is a widely implemented technique used to investigate large-scale topology in the human brain during health and disease. Studies in mice provide additional advantages, including the possibility to flexibly modulate the brain by pharmacological or genetic manipulations in combination with high-throughput functional connectivity (FC) investigations. Pharmacological modulations that target specific neurotransmitter systems, partly mimicking the effect of pathological events, could allow discriminating the effect of specific systems on functional network disruptions. The current study investigated the effect of cholinergic and serotonergic antagonists on large-scale brain networks in mice. The cholinergic system is involved in cognitive functions and is impaired in, e.g., Alzheimer’s disease, while the serotonergic system is involved in emotional and introspective functions and is impaired in, e.g., Alzheimer’s disease, depression and autism. Specific interest goes to the default-mode-network (DMN), which is studied extensively in humans and is affected in many neurological disorders. The results show that both cholinergic and serotonergic antagonists impaired the mouse DMN-like network similarly, except that cholinergic modulation additionally affected the retrosplenial cortex. This suggests that both neurotransmitter systems are involved in maintaining integrity of FC within the DMN-like network in mice. Cholinergic and serotonergic modulations also affected other functional networks, however, serotonergic modulation impaired the frontal and thalamus networks more extensively. In conclusion, this study demonstrates the utility of pharmacological rsfMRI in animal models to provide insights into the role of specific neurotransmitter systems on functional networks in neurological disorders.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Cholinergic and serotonergic modulations differentially affect large-scale functional networks in the mouse brain1501715421PapanikolaouKLLS20153APapanikolaouGAKelirisSLeeNKLogothetisSMSmirnakis2015-07-00Epub aheadNeuroImageThere is extensive controversy over whether the adult visual cortex is able to reorganize following visual field loss (scotoma) as a result of retinal or cortical lesions. Functional magnetic resonance imaging (fMRI) methods provide a useful tool to study the aggregate receptive field properties and assess the capacity of the human visual cortex to reorganize following injury. However, these methods are prone to biases near the boundaries of the scotoma. Retinotopic changes resembling reorganization have been observed in the early visual cortex of normal subjects when the visual stimulus is masked to simulate retinal or cortical scotomas. It is not known how the receptive fields of higher visual areas, like hV5/MT +, are affected by partial stimulus deprivation. We measured population receptive field (pRF) responses in human area V5/MT + of 5 healthy participants under full stimulation and compared them with responses obtained from the same area while masking the left superior quadrant of the visual field (“artificial scotoma” or AS). We found that pRF estimations in area hV5/MT + are nonlinearly affected by the AS. Specifically, pRF centers shift towards the AS, while the pRF amplitude increases and the pRF size decreases near the AS border. The observed pRF changes do not reflect reorganization but reveal important properties of normal visual processing under different test-stimulus conditions.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Nonlinear population receptive field changes in human area V5/MT + of healthy subjects with simulated visual field scotomas1501715421OrtizRiosKDAASJKR20153MOrtiz-RiosPKuśmierekIDeWittIAArchakovFAAzevedoMSamsIJääskeläinenGAKelirisJPRauschecker2015-04-001139110Frontiers in NeuroscienceUsing functional magnetic resonance imaging in awake behaving monkeys we investigated how species-specific vocalizations are represented in auditory and auditory-related regions of the macaque brain. We found clusters of active voxels along the ascending auditory pathway that responded to various types of complex sounds: inferior colliculus (IC), medial geniculate nucleus (MGN), auditory core, belt, and parabelt cortex, and other parts of the superior temporal gyrus (STG) and sulcus (STS). Regions sensitive to monkey calls were most prevalent in the anterior STG, but some clusters were also found in frontal and parietal cortex on the basis of comparisons between responses to calls and environmental sounds. Surprisingly, we found that spectrotemporal control sounds derived from the monkey calls (“scrambled calls”) also activated the parietal and frontal regions. Taken together, our results demonstrate that species-specific vocalizations in rhesus monkeys activate preferentially the auditory ventral stream, and in particular areas of the antero-lateral belt and parabelt.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published9Functional MRI of the vocalization-processing network in the macaque brain1501715421LeePKS20143SLeeAPapanikolaouGAKelirisSMSmirnakis2015-02-00962015Journal of Visualized ExperimentsVisual cortex is retinotopically organized so that neighboring populations of cells map to neighboring parts of the visual field. Functional magnetic resonance imaging allows us to estimate voxel-based population receptive fields (pRF), i.e., the part of the visual field that activates the cells within each voxel. Prior, direct, pRF estimation methods1 suffer from certain limitations: 1) the pRF model is chosen a-priori and may not fully capture the actual pRF shape, and 2) pRF centers are prone to mislocalization near the border of the stimulus space. Here a new topographical pRF estimation method2 is proposed that largely circumvents these limitations. A linear model is used to predict the Blood Oxygen Level-Dependent (BOLD) signal by convolving the linear response of the pRF to the visual stimulus with the canonical hemodynamic response function. PRF topography is represented as a weight vector whose components represent the strength of the aggregate response of voxel neurons to stimuli presented at different visual field locations. The resulting linear equations can be solved for the pRF weight vector using ridge regression3, yielding the pRF topography. A pRF model that is matched to the estimated topography can then be chosen post-hoc, thereby improving the estimates of pRF parameters such as pRF-center location, pRF orientation, size, etc. Having the pRF topography available also allows the visual verification of pRF parameter estimates allowing the extraction of various pRF properties without having to make a-priori assumptions about the pRF structure. This approach promises to be particularly useful for investigating the pRF organization of patients with disorders of the visual system.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Topographical estimation of visual population receptive fields by fMRI1501715421BahmaniMLK20143HBahmaniYMurayamaNKLogothetisGAKeliris2014-09-009918PLoS ONEPrimary visual cortex (V1) was implicated as an important candidate for the site of perceptual suppression in numerous psychophysical and imaging studies. However, neurophysiological results in awake monkeys provided evidence for competition mainly between neurons in areas beyond V1. In particular, only a moderate percentage of neurons in V1 were found to modulate in parallel with perception with magnitude substantially smaller than the physical preference of these neurons. It is yet unclear whether these small modulations are rooted from local circuits in V1 or influenced by higher cognitive states. To address this question we recorded multi-unit spiking activity and local field potentials in area V1 of awake and anesthetized macaque monkeys during the paradigm of binocular flash suppression. We found that a small but significant modulation was present in both the anesthetized and awake states during the flash suppression presentation. Furthermore, the relative amplitudes of the perceptual modulations were not significantly different in the two states. We suggest that these early effects of perceptual suppression might occur locally in V1, in prior processing stages or within early visual cortical areas in the absence of top-down feedback from higher cognitive stages that are suppressed under anesthesia.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published7Binocular Flash Suppression in the Primary Visual Cortex of Anesthetized and Awake Macaques1501715421SchmidK20143MCSchmidGAKeliris2014-07-00718342–344Trends in Cognitive SciencesInvestigations of topographic cortical plasticity following peripheral nervous injury predominantly report receptive field (RF) shifts toward the intact periphery. A recent study on visual cortex plasticity following retinal lesions by Botelho et al. finds RF coverage of the lesion affected space when global retinotopic mapping strategies are used.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-342Filling-in versus filling-out: patterns of cortical short-term plasticity1501715421PapanikolaouKPSKPSBSLS20143APapanikolaouGAKelirisTDPapageorgiouYShaoEKrappEPapageorgiouKStinglABruckmannUSchieferNKLogothetisSMSmirnakis2014-04-0016111E1656–E1665Proceedings of the National Academy of Sciences of the United States of AmericaInjury to the primary visual cortex (V1) typically leads to loss of conscious vision in the corresponding, homonymous region of the contralateral visual hemifield (scotoma). Several studies suggest that V1 is highly plastic after injury to the visual pathways, whereas others have called this conclusion into question. We used functional magnetic resonance imaging (fMRI) to measure area V1 population receptive field (pRF) properties in five patients with partial or complete quadrantic visual field loss as a result of partial V1+ or optic radiation lesions. Comparisons were made with healthy controls deprived of visual stimulation in one quadrant [“artificial scotoma” (AS)]. We observed no large-scale changes in spared-V1 topography as the V1/V2 border remained stable, and pRF eccentricity versus cortical-distance plots were similar to those of controls. Interestingly, three observations suggest limited reorganization: (i) the distribution of pRF centers in spared-V1 was shifted slightly toward the scotoma border in 2 of 5 patients compared with AS controls; (ii) pRF size in spared-V1 was slightly increased in patients near the scotoma border; and (iii) pRF size in the contralesional hemisphere was slightly increased compared with AS controls. Importantly, pRF measurements yield information about the functional properties of spared-V1 cortex not provided by standard perimetry mapping. In three patients, spared-V1 pRF maps overlapped significantly with dense regions of the perimetric scotoma, suggesting that pRF analysis may help identify visual field locations amenable to rehabilitation. Conversely, in the remaining two patients, spared-V1 pRF maps failed to cover sighted locations in the perimetric map, indicating the existence of V1-bypassing pathways able to mediate useful vision.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Population receptive field analysis of the primary visual cortex complements perimetry in patients with homonymous visual field defects1501715421LeePLSK20133SLeeAPapanikolaouNKLogothetisSMSmirnakisGAKeliris2013-11-0081144–157NeuroImageWe introduce a new method for measuring visual population receptive fields (pRF) with functional magnetic resonance imaging (fMRI). The pRF structure is modeled as a set of weights that can be estimated by solving a linear model that predicts the Blood Oxygen Level-Dependent (BOLD) signal using the stimulus protocol and the canonical hemodynamic response function. This method does not make a priori assumptions about the specific pRF shape and is therefore a useful tool for uncovering the underlying pRF structure at different spatial locations in an unbiased way. We show that our method is more accurate than a previously described method (Dumoulin and Wandell, 2008) which directly fits a 2-dimensional isotropic Gaussian pRF model to predict the fMRI time-series. We demonstrate that direct-fit models do not fully capture the actual pRF shape, and can be prone to pRF center mislocalization when the pRF is located near the border of the stimulus space. A quantitative comparison demonstrates that our method outperforms the direct-fit methods in the pRF center modeling by achieving higher explained variance of the BOLD signal. This was true for direct-fit isotropic Gaussian, anisotropic Gaussian, and difference of isotropic Gaussians model. Importantly, our model is also capable of exploring a variety of pRF properties such as surround suppression, receptive field center elongation, orientation, location and size. Additionally, the proposed method is particularly attractive for monitoring pRF properties in the visual areas of subjects with lesions of the visual pathways, where it is difficult to anticipate what shape the reorganized pRF might take. Finally, the method proposed here is more efficient in computation time than direct-fit methods, which need to search for a set of parameters in an extremely large searching space. Instead, this method uses the pRF topography to constrain the space that needs to be searched for the subsequent modeling.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-144A new method for estimating population receptive field topography in visual cortex1501715421ShaoKPFZ20133YShaoGAKelirisAPapanikolaouMDFischerDZoborHJägleNKLogothetisSMSmirnakis2013-11-0010383456–3464European Journal of NeuroscienceThe visual field is retinotopically represented in early visual areas. It has been suggested that when adult primary visual cortex (V1) is deprived of normal retinal input it is capable of large-scale reorganisation, with neurons inside the lesion projection zone (LPZ) being visually driven by inputs from intact retinal regions. Early functional magnetic resonance imaging (fMRI) studies in humans with macular degeneration (MD) report > 1 cm spread of activity inside the LPZ border, whereas recent results report no shift of the LPZ border. Here, we used fMRI population receptive field measurements to study, for the first time, the visual cortex organisation of one macaque monkey with MD and to compare it with normal controls. Our results showed that the border of the V1 LPZ remained stable, suggesting that the deafferented area V1 zone of the MD animal has limited capacity for reorganisation. Interestingly, the pRF size of non-deafferented V1 voxels increased slightly (~20% on average), although this effect appears weaker than that in previous single-unit recording reports. Area V2 also showed limited reorganisation. Remarkably, area V5/MT of the MD animal showed extensive activation compared to controls stimulated over the part of the visual field that was spared in the MD animal. Furthermore, population receptive field size distributions differed markedly in area V5/MT of the MD animal. Taken together, these results suggest that V5/MT has a higher potential for reorganisation after MD than earlier visual cortex.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-3456Visual cortex organisation in a macaque monkey with macular degeneration1501715421MaierPTK20123AMaierTIPanagiotaropoulosNTsuchiyaGAKeliris2012-09-00263613Frontiers in Human NeuroscienceIn 1593, Neapolitan polymath Giambattista della Porta publicly lamented that he was unable to improve his impressive productivity (he had published in areas as diverse as cryptography, hydraulics, pharmacology, optics, and classic fiction). Della Porta was trying to read two books simultaneously by placing both volumes side-by-side, and using each eye independently. To his great surprise, his setup allowed him to only read one book at a time. This discovery arguably marks the first written account of binocular rivalry (Wade, 2000) – a perceptual phenomenon that more than 400 years later still both serves to intrigue as well as to illuminate the limits of scientific knowledge. At first glance, binocular rivalry is an oddball. In every day vision, our eyes receive largely matching views of the world. The brain combines the two images into a cohesive scene, and concurrently, perception is stable. However, when showing two very different images (such as two different books) to each eye, the brain resolves the conflict by adopting a “diplomatic” strategy. Rather than mixing the views of the two eyes into an insensible visual percept, observers perceive a dynamically changing series of perceptual snapshots, with one eye’s view dominating for a few seconds before being replaced by its rival from the other eye. With prolonged viewing of a rivalrous stimulus, one inevitably experiences a sequence of subjective perceptual reversals, separated by random time intervals, and this process continues for as long as the sensory conflict is present.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published2Introduction to research topic – binocular rivalry: a gateway to studying consciousness1501715421FischerZKSSJLS20123MDFischerDZoborGAKelirisYShaoMWSeeligerSHaverkampHJägleNKLogothetisSMSmirnakis2012-08-003125179194Documenta OphthalmologicaAnimal models are powerful tools to broaden our understanding in disease mechanisms and to develop future treatment strategies. Here we present detailed
structural and functional findings of a rhesus macaque suffering from a naturally occurring bilateral macular dystrophy (BMD), partial optic atrophy and corresponding
reduction of central V1 signals in visual fMRI experiments when compared to data in a healthy macaque (CTRL) of similar age. Fluorescence and indocyanine green
angiography showed reduced macular vascularization with significantly larger foveal avascular zones (FAZ) in the affected animal (FAZBMD = 8.85 mm2 vs. FAZCTRL =
0.32 mm2). Optical coherence tomography showed bilateral thinning of the macula within the FAZ (total retinal thickness, TRTBMD = 174 ± 9 μm) and partial optic nerve
atrophy when compared to control (TRTCTRL = 303 ± 45 μm). Segmentation analysis revealed that inner retinal layers were primarily affected (inner retinal thickness,
IRTBMD = 33 ± 9 μm vs. IRTCTRL = 143 ± 45 μm), while the outer retina essentially maintained its thickness (ORTBMD = 141 ± 7 μm vs. ORTCTRL = 160 ± 11 μm).
Accordingly, a strong central reduction in the multifocal electroretinography and a specific attenuation of cone-derived signals in Ganzfeld electroretinography was found, whereas rod function remained normal.
We provided detailed characterization of a primate macular disorder. This study aims to stimulate awareness and further investigation in primates with macular disorders
eventually leading to the identification of a primate animal model and facilitating the preclinical development of therapeutic strategies.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published15Detailed functional and structural characterization of a macular lesion in a rhesus macaque1501715421StoewerGKBLDS20123SStoewerJGoenseGAKelirisABartelsNKLogothetisJDuncanNSigala2012-01-0017113PLoS OnefMRI experiments with awake non-human primates (NHP) have seen a surge of applications in recent years. However, the standard fMRI analysis tools designed for human experiments are not optimal for analysis of NHP fMRI data collected at high fields. There are several reasons for this, including the trial-based nature of NHP experiments, with inter-trial periods being of no interest, and segmentation artefacts and distortions that may result from field changes due to movement. We demonstrate an approach that allows us to address some of these issues consisting of the following steps: 1) Trial-based experimental design. 2) Careful control of subject movement. 3) Computer-assisted selection of trials devoid of artefacts and animal motion. 4) Nonrigid between-trial and rigid within-trial realignment of concatenated data from temporally separated trials and sessions. 5) Linear interpolation of inter-trial intervals and high-pass filtering of temporally continuous data 6) Removal of interpolated data and reconcatenation of datasets before statistical analysis with SPM. We have implemented a software toolbox, fMRI Sandbox (http://code.google.com/p/fmri-sandbox/), for semi-automated application of these processing steps that interfaces with SPM software. Here, we demonstrate that our methodology provides significant improvements for the analysis of awake monkey fMRI data acquired at high-field. The method may also be useful for clinical applications with subjects that are unwilling or unable to remain motionless for the whole duration of a functional scan.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published12An Analysis Approach for High-Field fMRI Data from Awake Non-Human Primates1501715421StoewerGKBLDS2011_23SStoewerJGoenseGAKelirisABartelsNKLogothetisJDuncanNSigala2011-12-00102913901400Magnetic Resonance ImagingFunctional magnetic resonance imaging (fMRI) experiments with awake nonhuman primates (NHPs) have recently seen a surge of applications. However, the standard fMRI analysis tools designed for human experiments are not optimal for NHP data collected at high fields. One major difference is the experimental setup. Although real head movement is impossible for NHPs, MRI image series often contain visible motion artifacts. Animal body movement results in image position changes and geometric distortions. Since conventional realignment methods are not appropriate to address such differences, algorithms tailored specifically for animal scanning become essential. We have implemented a series of high-field NHP specific methods in a software toolbox, fMRI Sandbox (http://kyb.tuebingen.mpg.de/~stoewer/), which allows us to use different realignment strategies. Here we demonstrate the effect of different realignment strategies on the analysis of awake-monkey fMRI data acquired at high field (7 T). We show that the advantage of using a nonstandard realignment algorithm depends on the amount of distortion in the dataset. While the benefits for less distorted datasets are minor, the improvement of statistical maps for heavily distorted datasets is significant.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published10Realignment strategies for awake-monkey fMRI data150171542166833GAKelirisNKLogothetisASTolias2010-09-0037301235312365Journal of NeuroscienceThe role of primary visual cortex (area V1) in subjective perception has intrigued students of vision for decades. Specifically, the extent to which the activity of different types of cells (monocular versus binocular) and electrophysiological signals (i.e. local field potentials versus spiking activity) reflect perception is still debated. To address these questions we recorded from area V1 of the macaque using tetrodes during the paradigm of binocular flash suppression, where incongruent images presented dichoptically compete for perceptual dominance. We found that the activity of a minority (20%) of neurons reflect the perceived visual stimulus and these cells exhibited perceptual modulations substantially weaker in comparison to their sensory modulation induced by congruent stimuli. Importantly, perceptual modulations were found equally often for monocular and binocular cells, demonstrating that perceptual competition in V1 involves mechanisms across both types of neurons. The power of the local field pot
ential (LFP) also showed moderate perceptual modulations with similar percentages of sites showing significant effects across frequency bands (18-22%). The possibility remains that perception may be strongly reflected in more elaborate aspects of activity in V1 circuits (e.g. specific neuronal subtypes) or perceptual states might have a modulatory role on more intricate aspects of V1 firing patterns (e.g. synchronization), not necessarily altering the firing rates of single cells or the LFP power dramatically.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published12The Role of the Primary Visual Cortex in Perceptual Suppression of Salient Visual Stimuli150171542162573ASEckerPBerensGAKelirisMBethgeNKLogothetisASTolias2010-01-005965327584587ScienceCorrelated trial-to-trial variability in the activity of cortical neurons is thought to reflect the functional connectivity of the circuit. Many cortical areas are organized into functional columns, in which neurons are believed to be densely connected and to share common input. Numerous studies report a high degree of correlated variability between nearby cells. We developed chronically implanted multitetrode arrays offering unprecedented recording quality to reexamine this question in the primary visual cortex of awake macaques. We found that even nearby neurons with similar orientation tuning show virtually no correlated variability. Our findings suggest a refinement of current models of cortical microcircuit architecture and function: Either adjacent neurons share only a few percent of their inputs or, alternatively, their activity is actively decorrelated.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published3Decorrelated Neuronal Firing in Cortical Microcircuits1501715421150171882356143PBerensGAKelirisASEckerNKLogothetisASTolias2008-12-0022199207Frontiers in NeuroscienceExtra-cellular voltage fluctuations (local field potentials; LFPs) reflecting neural mass action are ubiquitous across species and brain regions. Numerous studies have characterized the properties of LFP signals in the cortex to study sensory and motor computations as well as cognitive processes like attention, perception and memory. In addition, its extracranial counterpart  the electroencelphalogram (EEG)  is widely used in clinical applications. However, the link between LFP signals and the underlying activity of local populations of neurons remains largely elusive. Here, we review recent work elucidating the relationship between spiking activity of local neural populations and LFP signals. We focus on oscillations in the gamma-band (30-90Hz) of the local field potential in the primary visual cortex (V1) of the macaque that dominate during visual stimulation. Given that in area V1 much is known about the properties of single neurons and the cortical architecture, it provides an excellent opportunity to
study the mechanisms underlying the generation of the local field potential.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published8Feature selectivity of the gamma-band of the local field potential in primate primary visual cortex1501715421150171882352053PBerensGAKelirisASEckerNKLogothetisASTolias2008-06-0022111Frontiers in Systems NeuroscienceThe local field potential (LFP), comprised of low-frequency extra-cellular voltage fluctuations, has been used extensively to study the mechanisms of brain function. In particular, oscillations in the gamma-band (3090 Hz) are ubiquitous in the cortex of many species during various cognitive processes. Surprisingly little is known about the underlying biophysical processes generating this signal. Here, we examine the relationship of the local field potential to the activity of localized populations of neurons by simultaneously recording spiking activity and LFP from the primary visual cortex (V1) of awake, behaving macaques. The spatial organization of orientation tuning and ocular dominance in this area provides an excellent opportunity to study this question, because orientation tuning is organized at a scale around one order of magnitude finer than the size of ocular dominance columns. While we find a surprisingly weak correlation between the preferred orientation of multi-unit activity and gamma-band LFP
recorded on the same tetrode, there is a strong correlation between the ocular preferences of both signals. Given the spatial arrangement of orientation tuning and ocular dominance, this leads us to conclude that the gamma-band of the LFP seems to sample an area considerably larger than orientation columns. Rather, its spatial resolution lies at the scale of ocular dominance columns.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published10Comparing the feature selectivity of the gamma-band of the local field potential and the underlying spiking activity in primate visual cortex1501715421150171882347883ASToliasASEckerAGSiapasAHoenselaarGAKelirisNKLogothetis2007-12-0069837803790Journal of NeurophysiologyUnderstanding the mechanisms of learning requires characterizing how the response properties of individual neurons and interactions across populations of neurons change over time. In order to study learning in-vivo, we need the ability to track an electrophysiological signature that uniquely identifies each recorded neuron for extended periods of time. We have identified such an extracellular signature using a statistical framework which allows quantification of the accuracy by which stable neurons can be identified across successive recording sessions. Our statistical framework uses spike waveform information recorded on a tetrodes four channels in order to define a measure of similarity between neurons recorded across time. We use this framework to quantitatively demonstrate for the first time the ability to record from the same neurons across multiple consecutive days and weeks. The chronic recording techniques and methods of analyses we report can be used to characterize the changes in brain circuits du
e to learning.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published10Recording Chronically from the same Neurons in Awake, Behaving Primates150171542142943JPfeufferAShmuelGAKelirisTSteudelHMerkleNKLogothetis2007-07-00625869882Magnetic Resonance Imagingnonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published13Functional MR imaging in the awake monkey: effects of motion on dynamic off-resonance and processing strategies150171542144333GAKelirisAShmuelS-PKuJPfeufferAOeltermannTSteudelNKLogothetis2007-07-00336550570NeuroImagenonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published20Robust Controlled Functional MRI in Alert Monkeys at High Magnetic Field: Effects of Jaw and Body Movements150171542144803ASToliasGAKelirisSMSmirnakisNKLogothetis2005-09-009811251125Nature Neurosciencenonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Reply to "Motion processing in macaque V4"150171542133263KMoutoussisGAKelirisZKourtziNKLogothetis2005-08-0017452231–2243Vision ResearchThe relationship between brain activity and conscious visual experience is central to our understanding of the neural mechanisms underlying perception. Binocularrivalry, where monocular stimuli compete for perceptual dominance, has been previously used to dissociate the constant stimulus from the varying percept. We report here fMRI results from humans experiencing binocularrivalry under a dichoptic stimulation paradigm that consisted of two drifting random dot patterns with different motion coherence. Each pattern had also a different color, which both enhanced rivalry and was used for reporting which of the two patterns was visible at each time. As the perception of the subjects alternated between coherent motion and motion noise, we examined the effect that these alternations had on the strength of the MR signal throughout the brain. Our results demonstrate that motionperception is able to modulate the activity of several of the visual areas which are known to be involved in motion processing. More specifically, in addition to area V5 which showed the strongest modulation, a higher activity during the perception of motion than during the perception of noise was also clearly observed in areas V3A and LOC, and less so in area V3. In previous studies, these areas had been selectively activated by motion stimuli but whether their activity reflects motionperception or not remained unclear; here we show that they are involved in motionperception as well. The present findings therefore suggest a lack of a clear distinction between ‘processing’ versus ‘perceptual’ areas in the brain, but rather that the areas involved in the processing of a specific visual attribute are also part of the neuronal network that is collectively responsible for its perceptual representation.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-2231A binocular rivalry study of motion perception in the human brain150171542133293ASToliasGAKelirisSMSmirnakisNKLogothetis2005-05-0058591593Nature NeuroscienceMacaque area V4 neurons are generally not selective for direction of motion, as judged from their response to directional stimuli presented after a baseline condition devoid of movement (classical paradigm). We used a motion-adaptation paradigm to investigate whether stimulation history influences direction-of-motion selectivity. We found that classically non-directional V4 neurons develop direction-of-motion selectivity after adaptation. This underscores the dynamic nature of functional cortical architecture.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published2Neurons in macaque area V4 acquire directional tuning after adaptation to motion stimuli1501715421PanagiotaropoulosLK20142TIPanagiotaropoulosNKLogothetisGAKelirisOxford University PressNew York, NY, USA2015-04-00Oxford Handbook of Computational Perceptual Organizationnonotspecifiedhttp://www.kyb.tuebingen.mpg.de/accepted0Neural approaches to perceptual organization1501715421OrtizAABLLK20157MOrtizFACAzevedoLCAzevedoDZBallaGLohmannNKLogothetisGAKelirisRio de Janeiro, Brazil2015-07-109th IBRO World Congress on Neuroscience (IBRO 2015)nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Dynamic Functional Connectivity Reflects Complex Audiovisual Scenes Changes during Cognitive Processing15017154211501718821AzevedoOABLLK20157FAzevedoMOrtiz-RiosLCAzevedoDBallaGLohmannNKLogothetisGAKelirisRio de Janeiro, Brazil2015-07-109th IBRO World Congress on Neuroscience (IBRO 2015)nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Eingenvector Centrality Mapping during Natural Viewing in the Macaque Brain15017154211501718821BahmaniLLK20157HBahmaniKJLakshminarasimhanNKLogothetisGAKelirisRio de Janeiro, Brazil2015-07-089th IBRO World Congress on Neuroscience (IBRO 2015)nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Spike-Field Coherence Reflects Perceptual State in Monkey Primary Visual Cortex1501715421BahmaniMLK2014_27HBahmaniYMurayamaNKLogothetisGAKelirisSchramberg, Germany2014-10-002415th Conference of Junior Neuroscientists of Tübingen (NeNa 2014)Primary visual cortex (V1) was implicated as an important candidate for the site of perceptual suppression in numerous psychophysical and imaging studies. However, neurophysiological results in awake monkeys provided evidence for competition mainly between neurons in areas beyond V1. In particular, only a moderate percentage of neurons in V1 were found to modulate in parallel with perception with magnitude substantially smaller than the physical preference of these neurons. It is yet unclear whether these small modulations are rooted from local circuits in V1 or influenced by higher cognitive states. To address this question we recorded multi-unit spiking activity and local field potentials in area V1 of awake and anesthetized macaque monkeys during the paradigm of binocular flash suppression. We found that a small but significant modulation was present in both the anesthetized and awake states during the flash suppression presentation. Furthermore, the relative amplitudes of the perceptual modulations were not significantly different in the two states. We suggest that these early effects of perceptual suppression might occur locally in V1, in prior processing
stages or within early visual cortical areas in the absence of top-down feedback from higher cognitive stages that are suppressed under anesthesia.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-24Binocular Flash Suppression in the Primary Visual Cortex of
Anesthetized and Awake Macaques1501715421AzevedoFLK20147FACAzevedoEFlorinNKLogothetisGAKelirisSantorini, Greece2014-06-0051AREADNE 2014: Research in Encoding and Decoding of Neural EnsemblesThe primate brain is a dynamic system interconnected by temporally correlated functional networks. The structure of this correlated activity depends on the brain’s internal state and on stimulus input. In the absence of external stimulation, functional networks of spontaneous activity,
i.e. the salience network, the executive control network and the default mode network, can be observed. Their origin and function are not well understood, but they could reflect
neural noise within anatomically connected areas or active mechanisms related to perception and awareness. On the other hand, when the brain is being stimulated, a different pattern of activity emerges. Exactly how this patiotemporal transition happens is still unclear.
The objective of this study is to characterize the dynamic changes of BOLD based functional connectivity between resting-state and natural-stimuli-driven networks in the awake monkey brain. Due to its high spatial resolution, BOLD-fMRI is a powerful tool to study large-scale
correlated brain network activity. We used a paradigm containing sequences of movie-clips with different contexts including natural and artificial environments as well as periods devoid of any visual stimulation (resting) in order to identify the global activation patterns reflecting
the interplay between different populations of neurons under these conditions.
For our experiments, two macaque monkeys (Macaca mulatta) were trained in a mock scanner to remain headposted and motionless in a custom-made fMRI chair while a RI-compatible
periscope presented a movie clip, a gray background (FOV 30 x 23, 60 Hz, eff. res. 530 400 fibers) or nothing. After the behavioral training was completed, the monkeys were scanned under the same conditions in a Bruker 4.7 T vertical MRI scanner with a custom-designed whole-head coil (single-shot GE-EPI, TR 1000 ms, TE 18 ms, 128 64 18 voxels, 1 1 2 mm).
Each run lasted 10 min (600 volumes). We collected 30 functional runs of resting state activity (without any visual stimulation) and 30 functional runs of stimulus driven activity (1 min of a natural movie presentation alternated with 1 min of gray background) for each monkey. All the volumes containing artifacts were pre-selected and excluded from the data analysis. For the visual stimulation condition, we selected the scans with strong visual activation based on a generalized linear model (GLM).
Functional connectivity data analysis (group-level ICA with 20 components) of the scans devoid of stimulation revealed resting-state networks consistent with previous reports in humans and monkeys (Mantini et al., 2013, J. Neurosci.). Furthermore, preliminary analysis of the scans with visual stimulation revealed components reflecting visually driven networks. Currently, we are employing the eigenvector centrality mapping (ECM), which is a parameter-free effective connectivity method (Lohmann et al., 2010, PLoS ONE) as well as models based dynamic causal modeling (DCM) (Friston et al., 2003, Neuroimage) to delineate differences across stimulation with different contexts and to characterize the physiological mechanisms behind the
transition of brain states.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-51Dynamics Changes of Bold Functional Connectivity during Natural Viewing in the Awake Macaque Brain1501715421KelirisSPLS20137GAKelirisYShaoAPapanikolaouNKLogothetisMSmirnakisSan Diego, CA, USA2013-11-1343rd Annual Meeting of the Society for Neuroscience (Neuroscience 2013)Receptive fields (RF) are a fundamental property characterizing sensory neurons. In the visual domain, numerous electrophysiological and computational studies established the spatio-temporal characteristics of RFs and modeled early visual neurons in primates as Gabor filters with well-defined properties. Recently functional magnetic resonance imaging (fMRI) techniques have been introduced to estimate aggregate (voxel-based) “population” receptive field (pRF) sizes in humans (Dumoulin SO, Wandell BA, 2008). Population receptive field estimates are a function of: 1) the receptive field properties of single units belonging to a voxel, and 2) the scatter in the location of receptive field centers across units. In this study, we estimate RF sizes by exploiting the spatial-frequency selectivity of visual RFs modeled as Gabor functions. Blood oxygen level dependent (BOLD) measurements were collected from humans fixating in the center of band-limited white noise stimuli presented in a block-design (12 seconds ON, 20 seconds OFF). In different blocks, we modulated the spatial frequency content of the stimuli by changing the size of the pixels of the white noise. We found that the BOLD signal amplitude in retinotopic visual cortex changes dramatically with different stimulation conditions in a way consistent with the spatial frequency sensitivity of the Gabor RF models. We modeled the BOLD signal of each voxel as a sum of Gabors with homogeneous orientation distribution followed by a compressive non-linearity and fit this model to our data. The standard deviation of the Gaussian envelope of the Gabor function provides an estimate of RF size. Estimates obtained this way were compared to pRF estimates derived from additional experiments with moving bar stimuli. RF size estimates obtained with our method increased linearly with eccentricity as expected, but were significantly smaller in comparison to standard pRF measurements (Dumoulin SO, Wandell BA, 2008). The reason is that our method is not sensitive to the receptive field scatter that happens between units belonging to the same voxel. Similar experiments performed in anesthetized macaques provided RF size estimates comparable to electrophysiological measurements of single-unit RFs. We conjecture that our method estimates for the fist time average single-unit RF sizes in the human.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Estimation of average single-unit receptive field size by fMRI1501715421LakshminarasimhanLK20137KLakshminarasimhanNKLogothetisGAKelirisSan Diego, CA, USA2013-11-1043rd Annual Meeting of the Society for Neuroscience (Neuroscience 2013)The phase of spikes in the gamma cycle has previously been shown to be modulated by stimulus orientation in macaque V1 (Vinck et al. 2010, Womelsdorf et al. 2012). These studies suggested that such stimulus-dependent phase shifts selectively facilitate the impact of neurons that fire earlier in the gamma cycle, on their targets. However, such phase coding schemes implicitly depend on the generation of a consistent gamma oscillation frequency across stimulus conditions. To test this, we examined whether the stimulus orientation and the eye of presentation affected the peak gamma frequency in V1. Two macaque monkeys were trained to passively fixate on a central spot, while one of two orthogonally oriented gratings was presented monocularly through a mirror stereoscope for a period of one second. In different trials either the eye of presentation or the orientation, were changed. Local field potential (LFP) signals recorded from 168 sites across multiple sessions were found to exhibit significant coherence with concurrently recorded single-unit spikes in the gamma frequency range. The power spectral density of each of those LFPs was fit as the sum of a power function and a gaussian function, and the center of the gaussian was taken as the peak gamma frequency. We found that, across sites the peak frequency varied between 30 Hz and 45 Hz in both monkeys. Moreover, within each site there was a significant shift in the peak frequency both with orientation (median shift ~1.99Hz) as well as the eye of presentation (median shift ~0.89Hz). There was no systematic relationship between the direction of shift and stimulus preference. Given that the orientation-dependent phase shifts reported earlier were of a very small magnitude (only a few degrees), it follows that such frequency changes could be detrimental for phase-shift coding schemes that rely on entrainment of multiple cortical areas to a single ‘clock-like’ signal. Alternatively, neural mechanisms implementing phase computations could be relatively invariant to frequency changes by using more intricate signal properties like instantaneous phase.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Stimulus-dependent gamma-frequency shifting in the macaque v11501715421ShaoKSPLS20137YShaoGAKelirisMCSchmidAPapanikolaouNKLogothetisSMSmirnakisSan Diego, CA, USA2013-11-0943rd Annual Meeting of the Society for Neuroscience (Neuroscience 2013)Visual receptive fields have dynamic properties and it has been reported that receptive field organization can change following chronic visual deprivation. We used 4.7 Tesla functional magnetic resonance imaging (fMRI) to study the visual cortex of two healthy adult macaque monkeys, and two monkeys with binocular central retinal lesions. FMRI experiments were performed under light remifentanyl induced anesthesia (Logothetis et al. Nat. Neurosci. 1999). Standard moving horizontal/vertical bar stimuli and expanding ring stimuli were presented to the subjects and the population receptive field (pRF) method (Dumoulin and Wandell, Neuroimage 2008) was used to measure retinotopic maps and pRF sizes in early visual areas. In general, there is good agreement between maps obtained by fMRI and previous results obtained by anatomical and physiological methods. FMRI pRF sizes and electrophysiology measurements in healthy animals show similar trends. For the monkeys with a photocoagulation induced retinal lesion, a former study has shown that the fMRI defined lesion projection zone (LPZ) border in V1 did not shift following the lesion (Smirnakis et al. Nature 2005). We reanalyzed these data using pRF methods and suggest that pRF size in the non-deafferented V1 showed little, if any, change on average. However, voxels inside the LPZ of areas V2/V3 do show visual modulation over time following the lesion, suggesting that area V2/V3 has more capacity for plasticity than area V1. Further investigation using fMRI and standard electrophysiology methods is in progress.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Population receptive field measurements in the visual cortex of macaque monkeys with and without retinal lesions1501715421BahmaniLK2013_27HBahmaniNKLogothetisGAKelirisTübingen, Germany2013-09-00127128Bernstein Conference 2013The primary visual cortex (V1) was implicated as an important candidate for the site of perceptual suppression in numerous psychophysical and imaging studies (Lehky, 1988; Blake, 1989; Polonsky et al., 2000; Tong and Engel, 2001). However, neurophysiological results in awake monkeys provided evidence for competition mainly between neurons in areas beyond V1 (Leopold and Logothetis, 1996; Sheinberg and Logothetis, 1997). In particular, only a moderate percentage of neurons in V1 was modulated in parallel with perception and the magnitude of their modulation was substantially smaller than the physical preference of these neurons (Keliris et al., 2010). It is yet unclear whether these small modulations are rooted in local circuits in V1 or influenced by higher cognitive states. To address this question we recorded multi-unit spiking activity and local field potentials in area V1 of awake and anesthetized macaque monkeys during the paradigm of binocular flash suppression. The results showed that the pattern of perceptual modulation of neurons in V1 under the conditions of general anesthesia is almost identical to those recorded from awake monkeys. This suggests a role of local processes in V1 in perceptual suppression. Alternatively, these modulations could be caused by feedback from higher areas independent of conscious state.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published1Effects of binocular flash suppression in awake and anesthetized macaque1501715421LiLK2013_27QLiNKLogothetisGAKelirisTübingen, Germany2013-09-00128129Bernstein Conference 2013Local measurements by small receptive fields induce ambiguous and noisy one-dimensional motion estimation. This problem can be overcome by selective integration or pooling over time and space to reconstruct the global pattern (Adelson & Movshon, 1982). However, it remains unclear if the local signals from intersections could influence the global pattern motion perception. Many studies used multiple apertures in order to investigate motion integration over space (Alais, Van der Smagt, Van den Berg, & Van de Grind, 1998; Maruya, Amano, & Nishida, 2010; Mingolla, Todd, & Norman, 1992; Takahashi, 2004), but none took this issue into consideration. Here we developed a novel stimulus and try to answer this question. We used a mask with multiple transparent apertures over a moving plaid. The plaid consisted of two overlapping moving gratings with directions 135° apart. The apertures were small (0.4°) and were placed in locations that allowed either only single contours (AP1) or intersections (AP2) to pass through. We hypothesized that if motion integration takes place only at higher stages with larger receptive fields, the probability of coherent pattern motion perception would not be affected by the relative ratios of the aperture types. Our results indicate that motion perception is largely affected by the ratio of aperture types. We conjecture that parallel processes at different stages are involved in motion integration.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published1Parallel processes may underly pattern motion perception1501715421BahmaniLK20137HBahmaniNKLogothetisGAKelirisBremen, Germany2013-08-0014536th European Conference on Visual Perception (ECVP 2013)The primary visual cortex (V1) was implicated as an important candidate for the site of perceptual suppression in numerous psychophysical and imaging studies (Lehky, 1988; Blake, 1989; Polonsky et al., 2000; Tong and Engel, 2001). However, neurophysiological results in awake monkeys provided evidence for competition mainly between neurons in areas beyond V1 (Leopold and Logothetis, 1996; Sheinberg and Logothetis, 1997). In particular, only a moderate percentage of neurons in V1 was modulated in parallel with perception and the magnitude of their modulation was substantially smaller than the physical preference of these neurons (Keliris et al., 2010). It is yet unclear whether these small modulations are rooted in local circuits in V1 or influenced by higher cognitive states. To address this question we recorded multi-unit spiking activity and local field potentials in area V1 of anesthetized macaque monkeys during the paradigm of binocular flash suppression. The results showed that the pattern of perceptual modulation of neurons in V1 under the conditions of general anesthesia is almost identical to those recorded from awake monkeys. This suggests a role of local processes in V1 in perceptual suppression. Alternatively, these modulations could be caused by feedback from higher areas independent of conscious state.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-145Effects of binocular flash suppression in the anesthetized macaque1501715421LiLK20137QLiNKLogothetisGAKelirisBremen, Germany2013-08-0014536th European Conference on Visual Perception (ECVP 2013)Local measurements by small receptive fields (RFs) in V1 are thought to induce ambiguous and noisy one-dimensional motion estimation. This necessitates integration at higher brain stages for computation of global pattern motion. Electrophysiological evidence from monkeys viewing plaid stimuli is consistent with this hypothesis finding a small percentage of cells in V1 responding to pattern motion but the percentage is increasing in higher motion responsive areas MT and MST. We conjectured that a subset of V1 RFs residing on specific stimulus features could directly respond to the pattern motion thus biasing motion integration at higher stages. We used a novel stimulus to mimic V1 RF responses to plaids. It comprised of a mask with multiple transparent apertures (0.4°) over a moving plaid. The aperture locations were chosen in advance to be of two types: AP1 were chosen to “see” only single grating components at any given time while AP2 were chosen to “see” only grating intersections. We manipulated the percentage of these two types in different trials to test how they influence motion perception. We found that the motion perception of subjects changes sigmoidally from 100% transparent when all apertures are AP1 to 100% coherent when all apertures are AP2.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-145Pattern motion signals from V1 receptive fields1501715421AzevedoALK20127FACAzevedoLACAzevedoNKLogothetisGKelirisSchramberg, Germany2012-11-003613th Conference of the Junior Neuroscientists of Tübingen (NeNA 2012)Attention is a cognitive function thought to enhance our ability to select, process, and perceive only a behaviorally relevant fraction of the immense sensory input impinging on our receptors (Knudsen, 2007). Early electrophysiological studies in primates demonstrate that attention can modulate substantially the firing rate of single cells in extrastriate visual areas but has no
or little impact in the primary visual cortex (Moran & Desimone, 1985). In contrast, attention has been linked to strong bloodoxygen-level-dependent (BOLD) signal modulations in human subjects (Gandhi et al., 1999). Our goal is to understand how selective visual spatial attention
modulates the neuronal activity in primary visual cortex (V1) and how these effects are reflected in the different signals (single unit activity, local field potentials, and BOLD). To this end, we have trained two rhesus macaques to perform an orientation-change detection task in high field fMRI scanners (4.7T, 7T) while we can simultaneously acquire high-resolution fMRI maps and electrophysiological signals. Preliminary results suggest that attention modulates the BOLD and electrophysiological signals in distinct ways.We are currently trying to address the layer specificity of the effects by using MRI compatible multicontact probes and implanted RF coils that provide ultra-high resolution maps of the fMRI activations.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-36Effects of visual attention on neural processing in Rhesus' V1 by simultaneous electrophysiology and BOLD-fMRI1501715421LiFLK20127QLiRWFlemingNKLogothetisGAKelirisMünchen, Germany2012-09-14190Bernstein Conference 2012Perceptual multi-stability is established when the brain fails to reach a single interpretation of the input from the external world. This issue intrigued scientific minds for more than two hundred years. This phenomenon has been found in vision (Leopold & Logothetis, 1999), audition (Repp, 2007), olfaction (Zhou & Chen, 2009) and speech (Warren & Gregory, 1958). Crucial features are similar within and across modalities (Schwarts et al., 2012).
In the visual modality, a number of ambiguous visual patterns have been described such as the Necker cube, motion plaids, and binocular rivalry. Multi-stable stimuli can provide unique insights into visual processing, as changes in perception are decoupled from changes in the stimulus. Understanding of how multi-stable perception occurs might help one to understand visual perception in general.
A key question in multi-stable perception is what the brain processes are responsible in the identification and alternation of the percepts. Some investigators suggest that both top-down and bottom-up processes are involved (García Pérez, 1989) but others argue that multi-stable perception does not need high-level processing but happens automatically as low-level competition between the stimulus features (Akman et al., 2009; Wilson et al, 2000). Furthermore, it is well known that changes in stimulus features can bias perception in one or another direction, (Klink, et al., 2012).
In order to explore this question, we used multi-stable motion stimuli and specifically moving plaids consisting of three superimposed gratings moving in equidistant directions (difference of 120 deg). These stimuli induce the perception of component and pattern motion simultaneously since any two component gratings bind together and are perceived to move in the opposite direction of the third grating component. We modulated properties of the stimuli such as grating speed and size and recorded the responses of human subjects reporting the direction of the single grating using one of three buttons for each direction. Preliminary results show that perceptual dominance is greatly affected by the selection of grating speeds. Grating size did not greatly change the predominance of the different gratings. We find that gratings with speed closer to physiological values have greater probability to be perceived and that gratings with similar speeds tend to group more often than gratings with different speeds. Further manipulations of other stimulus features like contrast and spatial frequency allow parametric variations of the relative probabilities of different interpretations. Our future goal is to use this information to built models of perceptual alternations using probabilistic inference.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-190Multi-Stable Visual Motion Perception15017154211501715422SmirnakisKSPL20127SMSmirnakisGAKelirisYShaoAPapanikolaouNKLogothetisNaples, FL, USA2012-08-00139712th Annual Meeting of the Vision Sciences Society (VSS 2012)Visual receptive fields have dynamic properties that may change with the conditions of visual stimulation or with the state of chronic visual deprivation. We used 4.7 Tesla functional magnetic resonance imaging (fMRI) to study the visual cortex of two normal adult macaque monkeys and one macaque with binocular central retinal lesions due to a form of juvenile macular degeneration (MD). FMRI experiments were performed under light remifentanyl induced anesthesia (Logothetis et al. Nat. Neurosci. 1999). Standard moving horizontal/vertical bar stimuli were presented to the subjects and the population receptive field (pRF) method (Dumoulin and Wandell, Neuroimage 2008) was used to measure retinotopic maps and pRF sizes in early visual areas. FMRI measurements of normal monkeys agree with published electrophysiological results, with pRF sizes and electrophysiology measurements showing similar trends. For the MD monkey, the size and location of the lesion projection zone (LPZ) was consistent with the retinotopic projection of the retinal lesion in early visual areas. No significant BOLD activity was seen within the V1 LPZ, and the retinotopic organization of the non-deafferented V1 periphery was regular without distortion. Interestingly, area V5/MT of the MD monkey showed more extensive activation than area V5/MT of control monkeys which had part of their visual field obscured (artificial scotoma) to match the scotoma of the MD monkey. V5/MT PRF sizes of the MD monkey were on average smaller than controls. PRF estimation methods allow us to measure and follow in vivo how the properties of visual areas change as a function of cortical reorganization. Finally, if there is time, we will discuss a different method of pRF estimation that yields additional information.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-1397Population receptive field measurements in macaque visual cortex1501715421BahmaniLK20127HBahmaniNKLogothetisGAKelirisNaples, FL, USA2012-08-0068312th Annual Meeting of the Vision Sciences Society (VSS 2012)Several imaging studies in humans have shown the involvement of a frontoparietal network of cortical areas in perceptual transitions during bistable perception. To investigate further the possible role of parietal visual areas in perceptual alternations, we recorded extracellular neural activity in the lateral intraparietal area (LIP) of the rhesus macaque. The subject was initially presented with congruent patterns to the two eyes. Then the stimulus was switched for either one or both eyes (binocular flash suppression versus physical alternation), both resulting in perception of the newly presented stimulus. The recorded cells typically showed an initial burst of activity at stimulus onsets as well as stimulus switches. In contrast to previous reports by a number of fMRI studies, we found strong transient activity during physical alternations at the single cell level. This signal was also present during binocular flash suppression but to a lesser extent. Importantly, the amplitude of the signal dropped substantially in control conditions where physical changes were introduced in the stimuli but did not induce concomitant changes in perception. The transient response of the recorded neurons was followed by a tonic response which exhibited independent dynamics. Interestingly, this sustained activity was significantly lower during incongruent versus congruent stimulation. We conjecture that areas at the high end of the dorsal pathway might be involved in multistable perception in a different way in comparison with feature and object selective areas of the ventral pathway. The transient signal recorded in LIP neurons during perceptual transitions could potentially trigger reorganization of activity in constellations of feature selective neurons in the ventral pathway. In addition, the suppression of the sustained activity in LIP during incongruent stimulation may reflect inhibitory processes involved in the resolution of conflict between the two stimuli or indicate a failure to bind the sensory input into a coherent percept.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-683The role of parietal visual cortex in perceptual transitions during bistable perception1501715421PapanikolaouKPSKSLS20127APapanikolaouGAKelirisDTPapageorgiouYShaoEKrappUSchieferNLogothetisSMSmirnakisSantorini, Greece2012-06-0076AREADNE 2012: Research in Encoding and Decoding of Neural EnsemblesCortical damage of the visual pathway as a result of stroke typically leads to a loss of conscious vision in the affected region of the contralateral visual hemifield (scotoma). The most common visual injury involves the primary visual cortex (V1), the major relay of visual information to the rest of the cortex. However, in spite of this, several higher visual areas have been shown to be modulated by visual stimuli presented inside the scotoma. This suggests that there are alternate pathways to transmit information from the retina to the cortex that bypass V1 and transmit information directly to extrastriate visual areas. A much debated issue is whether adult visual cortex is able to reorganize after injury, and if so, what is the extent and the mechanism of the observed reorganization. The purpose of this study is to map visual cortex organization after injury, gathering information about the role that specific networks of brain areas play in cortical reorganization and recovery. To this end, we use functional
magnetic resonance imaging (fMRI) methods to study several subjects with quadrandanopia and hemianopia and compare them to a group of normal controls. FMRI measurements were
obtained during the presentation of a moving bar stimulus, which traverses the visual field while the subjects are fixating. These measurements are used to estimate voxel based population receptive field centers and radii using a direct isotropic Gaussian method introduced by Dumoulin and Wandell (1). In select controls an area of the stimulus is obscured (“artificial scotoma”) to simulate as much as possible the real scotoma of each patient. Preliminary results suggest that receptive field measurements obtained both in patients and in subjects examined under the artificial scotoma condition differ from measurements obtained in controls. In general, there appear to be no significant retinotopic map alterations in the borders of early visual areas of patients suffering from cortical lesions. However, there are some differences in the organization of higher visual areas such as hV5/MT+ compared to those of normal subjects. This may in part reflect the fact that some of the input to hV5/MT+ receptive fields has been lost with the V1+ lesion, but there are also suggestions that V1 bypassing pathways contribute.
In addition, population receptive field size of some of the patients’ spared visual areas show deviations from the normal range of population receptive field sizes derived from the control subjects with and without the artificial scotoma condition.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-76Population Receptive Field Mapping in Human Subjects after Lesions of the Visual Pathway1501715421ShaoKPFNJALS20127YShaoGAKelirisAPapanikolaouDMFischerDNagyHJaegleMAugathNKLogothetisSMSmirnakisSantorini, Greece2012-06-0082AREADNE 2012: Research in Encoding and Decoding of Neural EnsemblesVisual receptive fields have dynamic properties that may change with the conditions of visual stimulation or with the state of chronic visual deprivation. We used 4.7 Tesla functional magnetic resonance imaging (fMRI) to study the visual cortex of two adult normal macaque monkeys and one with binocular central retinal lesions due to a formof juvenile macular degeneration (MD). FMRI experiments were performed under light remifentanyl induced anesthesia
(Logothetis, et al., Nature Neuroscience, 1999). Standard moving horizontal/vertical bar stimuli were presented to the subjects and the population receptive field (pRF) method (Dumoulin and Wandell, Neuroimage 2008) was used to measure retinotopic maps and pRF sizes in early visual areas. In addition we used a new spatiotemporal dynamic modulation method to measure pRF sizes as comparison. In general fMRI measurements from the normal monkeys agree
with electrophysiological results in the literature, with fMRI pRF sizes and electrophysiology measurements showing similar trends. For the MD monkey, the size and location of the fMRI defined lesion projection zone (LPZ) in early visual areas is consistent with the retinotopic projection
of the retinal lesion. No significant activity is found within V1 LPZ of the MD monkey, and the retinotopic organization of the non-deafferented V1 periphery is regular without distortion.
Higher level visual areas (V5/MT) of the MD monkey show more extensive activation than areas of control monkeys with an artificial scotoma (to obscure part of the stimuli from the visual field as a simulation of the real scotoma) of comparable size. PRF sizes in the nondeafferented
V5/MT of the MD monkey are on average slightly smaller than controls. Further investigation using fMRI and standard electrophysiology methods is in progress.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-82Population receptive field measurements in the visual cortex of macaque monkeys with and without retinal lesions1501715421PapanikolaouKSKPSLS20117APapanikolaouGAKelirisYShaoEKrappEPapageorgiouUSchieferNKLogothetisSMSmirnakisWashington, DC, USA2011-11-0041st Annual Meeting of the Society for Neuroscience (Neuroscience 2011)Damage to the primary visual cortex (V1) as a result of stroke typically leads to the inability to perceive visual stimuli in the affected region of the contralateral visual hemifield (scotoma). However, in spite of this, several higher visual areas have been shown to be modulated by visual stimuli presented inside the scotoma. A much debated issue is whether adult visual cortex is able to reorganize after injury, and if so, what is the extent and the mechanism of the observed reorganization. Here we use functional magnetic resonance imaging (fMRI) methods to study visual cortex reorganization after injury in adult human subjects. To this end we applied a method introduced by Dumoulin and Wandell (Dumoulin SO, Wandell BA, Population receptive field estimates in human visual cortex, Neuroimage 39, 2008), which uses functional magnetic resonance imaging (fMRI) to measure the aggregate receptive field properties of neuronal populations voxel by voxel in the visual cortex. FMRI measurements were obtained during the presentation of a moving bar stimulus which traversed the visual field while the subjects were fixating and these measurements were used to derive an estimate of the voxel based population receptive field centre and radius. We studied several subjects with quadrandanopsia and hemianopsia resulting from cortical lesions and compared them to the range of measurements obtained from a group of normal controls. In general, retinotopic maps in the patients’ spared early visual cortex appear to be consistent with retinotopic maps obtained in control subjects. The organization of higher level visual areas, such as V3a/b and MT show preliminary some differences compared to those of normal subjects. Also preliminary results on the population receptive field size of some of the patients’ spared visual areas show deviations from the normal range of population receptive field sizes derived from the control subjects. We are in the process of obtaining further measurements to confirm these findings and to assess to what degree they correspond to cortical reorganization.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Population receptive field mapping in human subjects with lesions of the visual cortex1501715421ShaoKPALS20117YShaoGAKelirisAPapanikolaouMAugathNKLogothetisSMSmirnakisWashington, DC, USA2011-11-0041st Annual Meeting of the Society for Neuroscience (Neuroscience 2011)Visual receptive fields have dynamic properties that may change with the conditions of visual stimulation or with the state of chronic visual deprivation. We used 4.7 Tesla functional magnetic resonance imaging (fMRI) to study the visual cortex of two adult normal macaque monkeys and one with binocular central retinal lesions due to a form of juvenile macular degeneration (D06). FMRI experiments were performed under light remifentanyl induced anesthesia (Logothetis et al. Nat. Neurosci. 1999). Standard moving horizontal/vertical bar stimuli were presented to the subjects and the population receptive field (pRF) method (Dumoulin and Wandell, Neuroimage 2008) was used to measure retinotopic maps and pRF sizes in early visual areas. In addition we used a new spatiotemporal dynamic modulation method to measure pRF sizes as comparison. In general fMRI measurements from the normal monkeys agree with electrophysiological results in the literature, with fMRI pRF sizes and electrophysiology measurements showing similar trends. For the macular degeneration monkey (D06), the size and location of the fMRI defined lesion projection zone in early visual areas is consistent with the retinotopic projection of the retinal lesion. No significant activity was found within V1 LPZ of D06, and the retinotopic organization of the non-deafferented V1 periphery is regular without distortion. Higher level visual areas of D06 (V5/MT) show more extensive activation than areas of control monkeys with an artificial scotoma (to obscure part of the stimuli from the visual field as a simulation of the real scotoma) of comparable size. PRF sizes in the non-deafferented V5/MT of monkey D06 are on average slightly smaller than controls. Further investigation using fMRI and standard electrophysiology methods is in progress.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Population receptive field measurements in visual cortex of macaque monkeys1501715421BahmaniLK20117HBahmaniNKLogothetisGAKelirisFreiburg, Germany2011-10-00123125Computational Neuroscience & Neurotechnology Bernstein Conference & Neurex Annual Meeting (BC11)When dissimilar images are presented to the two eyes, perception starts alternating spontaneously between each monocular view, a phenomenon called binocular rivalry (Leopold and Logothetis, 1999). Several imaging studies in humans have shown the involvement of a frontoparietal network of cortical areas in perceptual transitions during binocular rivalry (Lumer et al., 1998). Here we investigate the possible role of parietal visual areas in perceptual alternations during rivalry in the rhesus macaque. Neural activity in the lateral intraparietal area (LIP) was recorded extracellularly while the subject was presented dichoptically and asynchronously with two rivalrous patterns, resulting in flash suppression (Keliris et al., 2010). The paradigm ensures excellent control over the subject’s perceptual state.
Preliminary results confirm the transient change of brain activity around perceptual reversals at the single cell level. The recorded cells typically showed an initial burst of activity after the onset of a stimulus as well as at stimulus/perceptual changes, followed by a sustained response (Bisley, 2004). The transient response of recorded neurons has a short latency, lasts a few hundred milliseconds and is always positive while the sustained response is suppressive in some cells and excitatory in others. We speculate that these responses may reflect two separate underlying processes. The short latency response may reflect a fast sensory signal conveying the information in a bottom-up manner, while the sustained activity may represent top-down influences originating from higher areas in the prefrontal cortex. The functional magnetic resonance imaging (fMRI) studies performed previously could not dissociate these two tightly overlapping signals because of the poor temporal resolution of the technique.
Analysis of the firing rates of single and multi-units indicate that the transient part of the response predicts well the change in perception while the sustained activity does not show a significant correlation with perceptual state. This might be explained by the little selectivity of the sustained response of parietal neurons towards particular stimuli (Lehky and Sereno, 2007). It is believed that LIP neurons provide a representational map of saliency, integrating bottom-up and top-down information to guide the allocation of spatial attention (Bisley et al., 2011). We argue that the transient response of LIP neurons after perceptual switches is an indication for a role of this region in providing a change signal to higher areas. It is possible, that the intraparietal activation observed in humans around perceptual transitions may simply reflect the elevation of neural activity as a result of a novel percept rather than a causal role of the region in driving the switches. We are therefore planning to extend the binocular flash suppression paradigm to normal binocular rivalry and monitor the activity around spontaneous perceptual alternations in order to delineate what happens without any concomitant physical change in the stimulus.
Furthermore, local field potentials, temporal dynamics of single unit activity and synchronization between neurons might provide a better understanding of the top-down influences of prefrontal cortex especially during the sustained response. This analysis is currently in progress.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published2Neural correlates of binocular rivalry in parietal cortex1501715421BahmaniLK2011_27HBahmaniNKLogothetisGAKelirisHeiligkreuztal, Germany2011-10-002212th Conference of Junior Neuroscientists of Tübingen (NeNA 2011)When dissimilar images are presented to the two eyes, perception starts alternating spontaneously
between each monocular view, a phenomenon called binocular rivalry (Leopold and
Logothetis, 1999). Several imaging studies in humans have shown the involvement of a frontoparietal
network of cortical areas in perceptual transitions during binocular rivalry (Lumer
et al., 1998). Here we investigate the possible role of parietal visual areas in perceptual alternations
during rivalry in the rhesus macaque. Neural activity in the lateral intraparietal area
(LIP) was recorded extracellularly while the subject was presented dichoptically and asynchronously
with two rivalrous patterns, resulting in flash suppression (Keliris et al., 2010).
The paradigm ensures excellent control over the subjectâs perceptual state. Preliminary results
confirm the transient change of brain activity around perceptual reversals at the single
cell level. The recorded cells typically showed an initial burst of activity after the onset of a
stimulus as well as at stimulus/perceptual changes, followed by a sustained response (Bisley,
2004). The transient response of recorded neurons has a short latency, lasts a few hundred
milliseconds and is always positive while the sustained response is suppressive in some cells
and excitatory in others. We speculate that these responses may reflect two separate underlying
processes. The short latency response may reflect a fast sensory signal conveying
the information in a bottom-up manner, while the sustained activity may represent top-down
influences originating from higher areas in the prefrontal cortex. The functional magnetic resonance
imaging (fMRI) studies performed previously could not dissociate these two tightly
overlapping signals because of the poor temporal resolution of the technique. Analysis of the
firing rates of single and multi-units indicate that the transient part of the response predicts
well the change in perception while the sustained activity does not show a significant correlation
with perceptual state. This might be explained by the little selectivity of the sustained
response of parietal neurons towards particular stimuli (Lehky and Sereno, 2007). It is believed
that LIP neurons provide a representational map of saliency, integrating bottom-up
and top-down information to guide the allocation of spatial attention (Bisley et al., 2011). We
argue that the transient response of LIP neurons after perceptual switches is an indication
for a role of this region in providing a change signal to higher areas. It is possible, that the
intraparietal activation observed in humans around perceptual transitions may simply reflect
the elevation of neural activity as a result of a novel percept rather than a causal role of the
region in driving the switches. We are therefore planning to extend the binocular flash suppression
paradigm to normal binocular rivalry and monitor the activity around spontaneous
perceptual alternations in order to delineate what happens without any concomitant physical
change in the stimulus. Furthermore, local field potentials, temporal dynamics of single
unit activity and synchronization between neurons might provide a better understanding of
the top-down influences of prefrontal cortex especially during the sustained response. This
analysis is currently in progress.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-22Neural Correlates of Binocular Rivalry in Parietal Cortex1501715421PapanikolaouKSKSLS20117APapanikolaouGKelirisYShaoEKrappUSchieferNKLogothetisSSmirnakisHeiligkreuztal, Germany2011-10-003712th Conference of Junior Neuroscientists of Tübingen (NeNA 2011)Damage to the primary visual cortex (V1) as a result of stroke typically leads to the inability
to perceive visual stimuli in the affected region of the contralateral visual hemifield (scotoma).
However, in spite of this, several higher visual areas have been shown to be modulated by
visual stimuli presented inside the scotoma. A much debated issue is whether adult visual
cortex is able to reorganize after injury, and if so, what is the extent and the mechanism of the
observed reorganization. We use functional magnetic resonance imaging (fMRI) methods to
study visual cortex reorganization after injury in adult human subjects.To this end we applied
a method introduced by Dumoulin and Wandell (Dumoulin SO, Wandell BA, Population receptive
field estimates in human visual cortex, Neuroimage 39, 2008), which uses functional
magnetic resonance imaging (fMRI) to measure the aggregate receptive field properties of
neuronal populations voxel by voxel in the visual cortex. FMRI measurements were obtained
during the presentation of a moving bar stimulus which traversed the visual field while the
subjects were fixating and these measurements were used to derive an estimate of the voxel
based population receptive field center and radius. We studied several subjects with quadrandanopsia
and hemianopsia resulting from cortical lesions and compared them to the range of
measurements obtained from a group of normal controls. In general, retinotopic maps in the
patients’ spared early visual cortex appear to be consistent with retinotopic maps obtained
in control. subjects. The organization of higher level visual areas, such as V3a/b and MT
show preliminary some differences compared to those of normal subjects. Also preliminary
results on the population receptive field size of some of the patients’ spared visual areas show
deviations from the normal range of population receptive field sizes derived from the control
subjects. We are in the process of obtaining further measurements to confirm these findings
and to assess to what degree they correspond to cortical reorganization.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-37Population receptive field mapping in human subjects with visual cortical lesions150171542170647GAKelirisYShaoAPapanikolaouXPengNKLogothetisSMSmirnakisSan Diego, CA, USA2010-11-0040th Annual Meeting of the Society for Neuroscience (Neuroscience 2010)A fundamental problem of neuroscience is being able to understand the input-output relationship of early sensory areas. Central to this understanding is the notion of the receptive field (RF). Previous electrophysiological studies in the visual system of primates have demonstrated that receptive fields do not have fixed properties but are dynamically changing as a function of the stimulation conditions or behavioral tasks. In the human, aggregate (voxel-based) receptive field sizes (pRF) of early visual cortex were estimated by fMRI using standard retinotopic stimuli (Dumoulin SO, Wandell BA, Population receptive field estimates in human visual cortex, Neuroimage 39, 2008) but it would be desirable to obtain more detailed estimates of RF spatio-temporal properties. In this study, we have used flickering checkerboard stimuli (white noise) in order to measure the spatio-temporal response properties of population RFs by fMRI. Blood oxygen level dependent (BOLD) measurements were performed both in anesthetized macaques (4.7 Tesla vertical scanner) and awake-fixating human subjects (3 Tesla Siemens Trio). We found that the BOLD-signal amplitude in early visual cortex changes as a function of the checker-size of the stimulus and this can provide estimates of the central excitatory integration area as well as the surround suppression of the aggregate RFs. The estimates of the size of the central portion of the RFs were found to increase with eccentricity within each visual area as well as from lower to higher visual areas. The results were comparable to pRF-size estimates derived from additional experiments with classical retinotopic stimuli (expanding rings, rotating wedges, moving horizontal and vertical bars). Current work focuses on evaluating how RF parameter estimates change as a function of the temporal frequency and contrast of the stimuli. We believe that this method can provide robust estimates of RF parameters which can be used in longitudinal studies for the assessment of cortical reorganization and plasticity in patients suffering from retinal and cortical lesions.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Assessing the spatio-temporal dynamics of visual receptive fields by fMRI150171542170557ASEckerPBerensGAKelirisMBethgeNKLogothetisASToliasSan Diego, CA, USA2010-11-0040th Annual Meeting of the Society for Neuroscience (Neuroscience 2010)Correlated trial-to-trial variability in the activity of cortical neurons is thought to reflect the functional connectivity of the circuit. Many cortical areas are organized into functional columns, in which neurons are believed to be densely connected and share common input. Numerous studies report a high degree of correlated variability between nearby cells. We developed chronically implanted multi-tetrode arrays offering unprecedented recording quality to re-examine this question in primary visual cortex of awake macaques. We found that even nearby neurons with similar orientation tuning show virtually no correlated variability.
In a total of 46 recording sessions from two monkeys, we presented either static or drifting sine-wave gratings at eight different orientations. We recorded from 407 well isolated, visually responsive and orientation-tuned neurons, resulting in 1907 simultaneously recorded pairs of neurons. In 406 of these pairs both neurons were recorded by the same tetrode.
Despite being physically close to each other and having highly overlapping receptive fields, neurons recorded from the same tetrode had exceedingly low spike count correlations (rsc = 0.005 ± 0.004; mean ± SEM). Even cells with similar preferred orientations (rsignal > 0.5) had very weak correlations (rsc = 0.028 ± 0.010). This was also true if pairs were strongly driven by gratings with orientations close to the cells’ preferred orientations.
Correlations between neurons recorded by different tetrodes showed a similar pattern. They were low on average (rsc = 0.010 ± 0.002) with a weak relation between tuning similarity and spike count correlations (two-sample t test, rsignal < 0.5 versus rsignal > 0.5: P = 0.003, n = 1907).
To investigate whether low correlations also occur under more naturalistic stimulus conditions, we presented natural images to one of the monkeys. The average rsc was close to zero (rsc = 0.001 ± 0.005, n = 329) with no relation between receptive field overlap and spike count correlations. We obtained a similar result during stimulation with moving bars in a third monkey (rsc = 0.014 ± 0.011, n = 56).
Our findings suggest a refinement of current models of cortical microcircuit architecture and function: either adjacent neurons share only a few percent of their inputs or, alternatively, their activity is actively decorrelated.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Decorrelated neuronal firing in cortical microcircuits1501715421150171882370657YShaoGAKelirisAPapanikolaouDMFischerDNagyHJägleMWSeeligerMAugathNKLogothetisSMSmirnakisSan Diego, CA, USA2010-11-0040th Annual Meeting of the Society for Neuroscience (Neuroscience 2010)Macular degeneration (MD) is a common cause of human visual impairment. Typically MD deprives the foveal part of the primary visual cortex from retinal input. It has been reported that visual areas undergo extensive plastic reorganization in response to such deprivation (Baker et al., J. Neurosci. 2005), but this question remains not conclusively settled.
We used 4.7 Tesla functional magnetic resonance imaging (fMRI) to study the visual cortex of an adult macaque monkey with binocular central retinal lesions due to a form of juvenile MD. FMRI experiments were performed under light remifentanyl induced anesthesia. Standard moving horizontal/vertical bar stimuli were presented to the subject and the population receptive field (RF) method (Dumoulin and Wandell, Neuroimage 2008) was used to measure retinotopic maps and population receptive field sizes in early visual areas. RF size was plotted as a function of eccentricity in early visual areas. As expected, population based RFs increase in size as a function of eccentricity within each visual area, and as we move from lower to higher visual areas at a fixed eccentricity. In general, there is good agreement between maps obtained by fMRI and previous results obtained by anatomical and physiological methods.
The pattern of activity elicited in the MD monkey was compared to the pattern of activity elicited in two control monkeys. The primary visual cortex of the MD animal shows an extensive area devoid of BOLD (blood oxygen level dependent) activity that includes the fovea and roughly corresponds to the expected size of the retinal lesion projection zone (LPZ). Visually driven activity starts beyond the border of the calcarine sulcus, at approximately 9 degrees eccentricity a distance of ~ 36 mm from foveal V1 in this animal. RF size maps derived from non-deafferented cortex abutting the retinal LPZ were comparable to RF size maps derived from the corresponding area in control subjects. Further investigation using fMRI and standard electrophysiology methods is in progress.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Population receptive field mapping in a macaque monkey with macular degeneration150171542170637APapanikolaouGAKelirisXPengYShaoEKrappEPapageorgiouUSchieferNKLogothetisSMSmirnakisSan Diego, CA, USA2010-11-0040th Annual Meeting of the Society for Neuroscience (Neuroscience 2010)Damage to the primary visual cortex (V1) as a result of stroke or other brain diseases can lead to a loss of conscious vision in the contralateral visual hemifield. Cortical blindness affects many activities on a patient's daily life and is considered to be a heavy burden while there are few, if any, options for rehabilitation and recovery. A much debated issue is whether the visual cortex is able to reorganize after injury in adult human subjects, and if so, what may be the mechanism of reorganization. Here we apply an important new approach introduced by Dumoulin and Wandell (Doumoulin SO, Wandell BA, Population receptive field estimates in human visual cortex, Neuroimage 39, 2008), which uses functional magnetic resonance imaging (fMRI) to measure the aggregate receptive field properties of neuronal populations voxel by voxel in the visual cortex. The purpose of this study is to compare receptive field measurements in patients with cortical lesions with controls and to investigate whether these measurements change following injury. Patients were fixating in the magnet and fMRI measurements were obtained during the presentation of standard visual stimuli used in retinotopic mapping (rotating wedges, expanding rings, horizontally and vertically moving bars). The patient’s intact hemisphere, as well as normal subjects were used as controls. In some controls an area of the stimulus was obscured (“artificial scotoma”) to simulate as much as possible the real scotoma of the patients. Preliminary results suggest that receptive field measurements obtained in patients and in subjects examined under the artificial scotoma condition differ from measurements obtained in controls under the normal visual stimulation condition. We are in the process of obtaining further control tests and measurements to confirm these findings and to assess to what degree they correspond to cortical reorganization. Future research will focus on using these methods to study the capacity of various visual rehabilitation training methods to induce visual cortex reorganization.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Population receptive field mapping in human subjects with visual cortical lesions150171542168107ASEckerPBerensGAKelirisMBethgeNKLogothetisASToliasSantorini, Greece2010-06-0058AREADNE 2010: Research in Encoding And Decoding of Neural EnsemblesCorrelated trial-to-trial variability in the activity of cortical neurons is thought to reflect the
functional connectivity of the circuit. Many cortical areas are organized into functional columns,
in which neurons are believed to be densely connected and share common input. Numerous
studies report a high degree of correlated variability between nearby cells. We developed
chronically implanted multi-tetrode arrays offering unprecedented recording quality
to re-examine this question in primary visual cortex of awake macaques. We found that
even nearby neurons with similar orientation tuning show virtually no correlated variability.
In a total of 46 recording sessions from two monkeys, we presented either static or drifting
sine-wave gratings at eight different orientations. We recorded from 407 well isolated, visually
responsive and orientation-tuned neurons, resulting in 1907 simultaneously recorded
pairs of neurons. In 406 of these pairs both neurons were recorded by the same tetrode.
Despite being physically close to each other and having highly overlapping receptive fields,
neurons recorded from the same tetrode had exceedingly low spike count correlations (rsc =
0.005 ± 0.004; mean ± SEM). Even cells with similar preferred orientations (rsignal > 0.5) had
very weak correlations (rsc = 0.028 ± 0.010). This was also true if pairs were strongly driven
by gratings with orientations close to the cells’ preferred orientations.
Correlations between neurons recorded by different tetrodes showed a similar pattern. They
were low on average (rsc = 0.010 ± 0.002) with a weak relation between tuning similarity
and spike count correlations (two-sample t test, rsignal < 0.5 versus rsignal > 0.5: P = 0.003, n =
1907).
To investigate whether low correlations also occur under more naturalistic stimulus conditions,
we presented natural images to one of the monkeys. The average rsc was close to zero
(rsc = 0.001 ± 0.005, n = 329) with no relation between receptive field overlap and spike
count correlations. We obtained a similar result during stimulation with moving bars in a
third monkey (rsc = 0.014 ± 0.011, n = 56).
Our findings suggest a refinement of current models of cortical microcircuit architecture and
function: either adjacent neurons share only a few percent of their inputs or, alternatively,
their activity is actively decorrelated.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-58Decorrelated Firing in Cortical Microcircuits15017154211501718823KapoorWPKL20097VKapoorKWhittingstallTPanagiotaropoulosGKelirisNKLogothetisEllwangen, Germany2009-11-002210th Conference of Junior Neuroscientists of Tübingen (NeNa 2009)When disparate visual stimuli are presented to corresponding retinal locations, perception fluctuates between the presented stimuli. This phenomenon, called binocular rivalry, is an exquisite tool to dissociate sensory stimulation from visual perception. It has therefore been extensively used for studying the neural correlates of visual awareness. Initial theories have
tried to explain binocular rivalry by hypothesizing the resolution of competition in V1 through inhibitory interactions between monocular neurons. However, inter-ocular switch rivalry, a paradigm where the rivaling stimuli are rapidly exchanged between the eyes also results in stable percepts that span several swaps of the visual stimuli. This has demonstrated that competition also involves higher-level stimulus representations, and not just eye based sensory information. In this study, we compared the electrophysiological correlates underlying
stable visual percepts during inter-ocular switch and binocular rivalry. Delineating the differences
and similarities between the two paradigms of rivalry will provide us with valuable information on the nature of competition during incongruent visual stimulation. We recorded EEGs while human subjects experienced inter-ocular switch and classical binocular rivalry elicited with dichoptic presentation of orthogonally oriented sinusoidal gratings. The subjects reported their percepts via button presses. We extracted and analyzed trials, where subjects
reported at least one second long stable percepts. During this time window, we assessed the normalized spectrogram to visualize mean event-related changes in spectral power across a broad frequency range (1 - 45 Hz). We observed a strong and sustained increase in spectral power between 12 - 30 Hz across the two conditions approximately 300 ms following the reported perceptual switch. Low Resolution Brain Electromagnetic Tomography (LORETA) was used to localize the cortical sources of the observed changes. The maps of the localized cortical sources of this increase in the spectral power during inter-ocular switch and binocular
rivalry were remarkably similar and showed no significant differences. We therefore propose that both types of rivalry have similar EEG correlates in the 12 - 30 Hz frequency band during a stable visual percept.
22nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-22Comparing inter-ocular switch and classical binocular rivalry in the human brain using EEG150171542162867VKapoorKWhittingstallTPanagiotaropoulosGAKelirisNKLogothetisChicago, IL, USA2009-10-0039th Annual Meeting of the Society for Neuroscience (Neuroscience 2009)When disparate visual stimuli are presented to corresponding retinal locations in the two eyes, perception fluctuates between the presented stimuli. This phenomenon, called binocular rivalry, is an exquisite tool to dissociate sensory stimulation from visual perception. It has therefore been extensively used for studying the neural correlates of visual awareness. Initial theories have tried to explain binocular rivalry by hypothesizing the resolution of competition in V1 through inhibitory interactions between monocular neurons. However, inter-ocular switch rivalry, a paradigm where the rivaling stimuli are rapidly exchanged between the eyes also results in stable percepts that span several swaps of the visual stimuli. This has demonstrated that competition also involves higher level stimulus representations, and not just eye based sensory information. In this study, we compared the electrophysiological correlates underlying stable visual percepts during inter-ocular switch and classical binocular rivalry. Delineating the differences and similarities between the two paradigms of rivalry will provide us with valuable information on the nature of competition during incongruent visual stimulation. We recorded EEGs while human subjects experienced inter-ocular switch and classical binocular rivalry elicited with dichoptic presentation of orthogonally oriented sinusoidal gratings. The subjects reported their percepts via button presses. We extracted and analyzed trials, where subjects reported at least one second long stable percepts. During this time window, we assessed the normalized spectrogram to visualize mean event-related changes in spectral power across a broad frequency range (1-45 Hz). We observed a strong and sustained increase in spectral power between 12-30 Hz across the two conditions approximately 300 ms following the reported perceptual switch. Low Resolution Brain Electromagnetic Tomography (LORETA) was used to localize the cortical sources of the observed changes. The maps of the localized cortical sources of this increase in the spectral power during inter-ocular switch and binocular rivalry were remarkably similar and showed no significant differences. We therefore propose that both types of rivalry have similar EEG correlates in the 12-30 Hz frequency band during a stable visual percept.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Comparing inter-ocular switch and classical binocular rivalry in the human brain using eeg150171542162907GAKelirisASToliasNKLogothetisChicago, IL, USA2009-10-0039th Annual Meeting of the Society for Neuroscience (Neuroscience 2009)Understanding the neural underpinnings of conscious perception has long intrigued the students of the brain from philosophers to modern neuroscientists. In the visual domain, the primary visual cortex (V1) is by far the most extensively studied cortical area. It entails the main gateway of visual information to higher cortical areas and we understand a lot about its function in sensory processing. Nevertheless, the role of V1 in perceptual awareness remains intensely debated. Under certain stimulus conditions perception alternates between two or multiple stimulus interpretations. Notably such perceptual alternations happen while the sensory input is kept constant, offering thus a clear dissociation of sensory stimulation and subjective awareness. A celebrated example of such a perceptual phenomenon is binocular rivalry (BR). It involves the dichoptic presentation of two different stimuli at corresponding retinal locations and results in the perceptual suppression of one of the two stimuli at different times. A slight variant of BR, binocular flash suppression (BFS), ensures excellent control over the subjects’ perceptual state by intermittent presentation of monocular and binocular stimuli. We have trained rhesus macaques to report their perception during BFS and BR to study the effects of perceptual suppression in V1. We have recorded the spiking activity of a large number of well isolated single units (SUA) and acquired simultaneous local field potentials (LFPs) during the dichoptic presentation of orthogonal orientation gratings. We found that during BFS, 20% of the single units modulated their activities in consonance with the perceptual state. Furthermore, the magnitude of the perceptual effect was small (15%) in comparison to the sensory preference of the neurons. Analysis of the ocularity preferences demonstrated that both monocular and binocular classes of cells show perceptual modulations with equal probability. In addition, cells modulating during perceptual suppression encode information matching their sensory preferences and therefore can be used for decoding both the orientation and/or the eye of presentation of the perceived grating. Results of the LFPs were very similar to the single units showing a similar percentage of sites modulating with perception in all analyzed frequency bands. We conclude that footprints of perception are evident in both the SUA and LFP signals in V1 but in a much smaller degree than their corresponding sensory selectivity. Perceptual states might have a modulatory role on more intricate aspects of V1 firing patterns, not necessarily altering the firing rates of single cells or the LFP power dramatically.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Primary visual cortex contributions in perceptual supppression1501715421PanagiotaropoulosKKTL20097TPanagiotaropoulosVKapoorGAKelirisAToliasNKLogothetisRhodos, Greece2009-09-0041st European Brain and Behaviour Society MeetingBinocular rivalry (BR) has been successfully combined with extracellular electrophysiological recordings in awake, behaving macaques to study the cortical mechanisms of subjective visual perception. Here we used binocular flash suppression (BFS), a highly controlled variant of BR, to explore the neuronal correlates of visual awareness in the inferior prefrontal convexity (icPFC) of the macaque brain while simultaneously recording multi unit activity (MUA) and local field potentials (LFP). We found that MUA was perceptually modulated in 67% of the visually selective recording sites. During BFS in 92% of MUA modulated sites we observed higher firing rates when the preferred stimulus was perceived. An explicit representation of the perceptually dominant stimulus was also provided by the power modulation of high frequency LFP’s only at the MUA modulated sites. Specifically, sensory selectivity of the LFP power increased as a function of frequency with the highest selectivity observed between 150 and 450Hz. The same pattern in LFP power selectivity was observed when the preferred stimulus was perceived during BFS. A correlation analysis between MUA and LFP power selectivity showed significant correlation in sensory selectivity for frequencies >60Hz that saturated at 150Hz and followed the same pattern during BFS.
While spikes measure cortical output, LFP’s are thought to reflect input and intracortical processing in a given cortical area. According to this scheme our results suggest that icPFC sites providing perceptually modulated output are also the sites that receive and process a representation of the perceived stimulus during BFS. Inferior temporal cortex (IT) output is also known to reflect the perceived stimulus during ambiguous visual stimulation and could thus be the source of the modulated icPFC input reflected in the LFP’s. Our results suggest a highly organized network involving IT and icPFC that mediates visual awareness during subjective visual perception.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0High frequency local field potentials and multi unit activity reflect visual awareness in the macaque prefrontal cortex150171542161777SStoewerJDuncanABartelsGAKelirisNKLogothetisNSigalaCambridge, UK2009-03-0021st Cambridge Neuroscience Seminar: New Approaches in Neuroscience (CNS 2009)nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0SANDBOX, an interactive fMRI data visualization toolbox150171542154437TPanagiotaropoulosVKapoorGAKelirisAToliasNKLogothetisGeneva, Switzerland2008-07-006th Forum of European Neuroscience (FENS 2008)Human fMRI studies during binocular rivalry have demonstrated an involvement of prefrontal cortex (PFC) in the processing of subjective visual perception. In this study we used binocular flash suppression, a version of binocular rivalry that permits the robust induction of a visual percept, to study the neuronal correlates of visual awareness in the macaque prefrontal cortex (PFC) and specifically in the inferior prefrontal convexity. We found that the firing rate of almost 70% of the visually selective neurons closely followed the induced visual percept. This percentage is significantly higher than the respective percentage of perceptually modulated cells found in the striate and extrastriate visual cortex (V1, V2 and V4) but smaller than that found in the inferior temporal cortex (IT) (almost 90%). Interestingly, we observed that the neuronal responses following a perceptual alternation were transient, similar to the transient BOLD response observed during perceptual transitions in the human binocular rivalry fMRI studies. Our finding provides further evidence in support of a role of higher brain areas in processing an explicit perceptual representation during ambiguous visual stimulation. In addition, it points to a potential neuronal network consisting of perceptually modulated cells in IT and PFC that process an explicit representation of a visual percept. The existence of such a network is not surprising since area TE of inferior temporal cortex is anatomically connected to the inferior convexity (areas 12/45) through feedforward and feedback pathways.
Finally, in an effort to explore whether the perceptual modulation observed in primary visual cortex (V1) is influenced by a feedback signal from PFC we will also present data from simultaneous PFC and V1 neurophysiological recordings during binocular flash suppression.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Neurophysiological substrates of visual awareness in the macaque prefrontal cortex150171542155117GAKelirisASToliasNKLogothetisGeneva, Switzerland2008-07-006th Forum of European Neuroscience (FENS 2008)When two incongruent stimuli are presented simultaneously at corresponding retinal locations in the two eyes, one typically experiences a perceptual alternation of the two stimuli; a phenomenon known as binocular rivalry. Binocular flash suppression (BFS) is a variant of binocular rivalry and refers to the sudden and persistent perceptual suppression resulting when two rivalrous patterns are presented dichoptically and asynchronously to the two eyes. Under these conditions, the latter pattern dominates perceptually over the first. The binocular flash suppression paradigm ensures excellent control over the subject’s perceptual state without the need for subjective reports which involve decision making, action preparation and action execution. The role of primary visual cortex (V1) in perceptual suppression remains controversial. In this study, we assessed quantitatively the effects of perceptual suppression on neural activity in V1 of the macaque using BFS. We have analyzed both the spiking activity of a large number of single neurons (SUA) and different frequency bands of the local field potentials (LFPs). The main result for SUA was that only a small minority (~20%) modulates in consonance with the perceptual suppression of static orientation gratings. Furthermore, the magnitude of the perceptual effect was small (~15%) in comparison to the sensory preference of the neurons. LFPs showed comparable percentages. The amplitude of LFP modulations was independent of frequency although gamma frequencies showed greater selectivity during physical alternation of the stimuli. Our results provide evidence against the hypothesis that competition is happening at the level of monocular neurons at the input layers of primary visual cortex.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0The Role of Primary Visual Cortex (V1) in Perceptual Suppression150171542154427TPanagiotaropoulosVKapoorGAKelirisAToliasNKLogothetisSantorini, Greece2008-06-0080AREADNE 2008: Research in Encoding and Decoding of Neural Ensemblesnonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-80Single units reflect visual awareness in the macaque prefrontal cortex150171542155107GAKelirisASToliasNKLogothetisSantorini, Greece2008-06-0061AREADNE 2008: Research in Encoding and Decoding of Neural EnsemblesUnder certain stimulus conditions a single interpretation of the external world cannot be
unambiguously designated. When the brain is presented with such stimuli typically only one
possible interpretation is perceived and after a few seconds the percept switches abruptly to
another. Notably such perceptual alternations happen while the sensory input is kept
constant, offering thus a clear dissociation of sensory stimulation and subjective awareness.
A celebrated example of such a perceptual phenomenon is binocular rivalry (BR). It involves
alternations of visual perception between two different images presented dichoptically at corresponding
retinal locations. Based on many psychophysical studies over decades the primary
visual cortex (V1) was implicated as an important candidate for the site of perceptual suppression.
However, the first neurophysiological evidence performed in monkeys did not corroborate
this but instead found only a small percentage of neurons modulating their activity with
the subjective awareness reported by the animals. On the contrary, studies using human functional
magnetic resonance imaging (fMRI), have found V1 to be modulating to a large extent,
creating an apparent controversy. Therefore, the role of primary visual cortex (V1) in subjective
perception remains controversial.
In this study, we studied the effects of perceptual suppression on neural activity in V1 of the
macaque. We have used the binocular flash suppression (BFS) paradigm, a variant of BR
which ensures excellent control over the subject’s perceptual state. We have recorded the
spiking activity of a large number of well isolated single units (SUA) and acquired simultaneous
local field potentials (LFPs) during the dichoptic presentation of orthogonal orientation
gratings. Our design enabled us to determine a) which neurons and LFP bands are correlated
with the percept and b) how this is related to their orientation and ocularity preferences.
We find that only a small minority of about 20% of the single units modulate in consonance
with the perceptual suppression. Furthermore, the magnitude of the perceptual effect was
small (~15%) in comparison to the sensory preference of the neurons. Results of the LFPs
were very similar to the single units showing a similar percentage of sites modulating with
perception. Analysis of the orientation and ocularity preference of neurons did not show a particular
class of cells to be having a greater probability to show perceptual modulations.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-61The Role of Primary Visual Cortex in Perceptual Awareness150171542155127GAKelirisASToliasNKLogothetisPittsburgh, PA, USA2008-02-00First Annual inter-Science of Learning Center (iSLC): Student and Postdoc Conferencenonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Binocular Flash Suppression in area V1 of the macaque150171542145917PBerensASEckerGAKelirisNKLogothetisASToliasSan Diego, CA, USA2007-11-0037th Annual Meeting of the Society for Neuroscience (Neuroscience 2007)The local field potential (LFP) and, in particular, the gamma-band frequency range (30-90 Hz) have recently received much attention, as numerous studies have shown correlations between LFP and sensory, motor and cognitive variables in various cortical regions. However, the extent to which it reflects the activity of local populations of neurons remains elusive. The issue of spatial scale is central for understanding the origins of the LFP and how this signal can be used to study the functional organization of the brain.
We addressed this question by simultaneously recording multi-unit spiking activity (MUA) and LFP from the primary visual cortex (V1) of awake, behaving macaques using arrays of tetrodes. Oriented gratings were used for visual stimulation, applied either binocular or monocular. The columnar organization of stimulus orientation and ocularity in V1 provides an excellent opportunity to study the spatial precision of the LFP signal, because neurons with similar orientation preference are organized at the fine spatial scale of cortical microcolumns (50-100 μm), whereas ocular dominance columns span around 450 μm.
As shown before, we find that the increase of LFP gamma-band power is a function of orientation and ocularity of the stimulus. However, the power of the gamma-band contains much less information about the orientation of the stimulus than the MUA recorded at the same site. The average discriminability d' between preferred and orthogonal orientation was 2.46±0.15 for MUA and 1.01±0.05 for LFP (mean ±std). Moreover, we find only a weak correlation between the preferred orientation of the MUA tuning function and that of the LFP (r=0.21, p<0.05). In contrast, we find a strong correlation between the preferred ocularity of the two signals (r=0.53, p<1e-9).
We therefore conclude that the gamma-power of the LFP does not reflect well the local activity on the scale of orientation columns but does capture the ocular dominance structure of V1. We suggest that gamma-band activity is generated by ensembles of neurons larger than 50-100 μm. In agreement with a previous study (Liu & Newsome, 2006) we find that it more likely resembles the activity of neurons from an area spanning a few hundred micrometers.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0On the spatial scale of the local field potential - orientation and ocularity tuning of the local field potential in the primary visual cortex of the macaque150171542147337ASEckerAGSiapasAHoenselaarPBerensGAKelirisNKLogothetisASToliasSan Diego, CA, USA2007-11-0037th Annual Meeting of the Society for Neuroscience (Neuroscience 2007)Understanding the mechanisms of learning and memory consolidation requires characterizing how the response properties of individual neurons and interactions across populations of neurons change over time, during periods spanning multiple days.
We used multiple chronically implanted tetrodes to record single unit activity from area V1 of the awake, behaving macaque and developed a method to quantitatively determine recording stability. Our method is based on a statistical framework which uses similarity of action potential waveforms to detect stable recordings given a pre-defined type I error rate. The similarity measure that was used takes into account both the shape of the action potential waveform and the amplitude ratio across channels, which depends on the location of the neuron relative to the tetrode.
271 well-isolated single units were recorded from 7 tetrodes during two periods of up to 23 days. We computed the distribution of pairwise similarities of average waveforms recorded on consecutive recording sessions during the first 34 days after implantation of the chronic drive. During this period, there was no recording stability due to regular adjustments of the tetrodes. We used this distribution as an empirical null distribution for hypothesis testing.
Using this statistical procedure and a type I error rate of alpha = 0.05, we find that of all single units recorded on a given day, 51% could be recorded for at least 2 days, 40% for at least 3 days, and 25% for at least 7 days.
In addition, we adapted a recently proposed multivariate statistical test (Gretton et al., 2007) to test whether the waveforms obtained at consecutive days come from the same underlying probability distribution. Using this test we obtained qualitatively similar results.
To validate these results, we compared orientation tuning functions of neurons that were tracked across days. Consistent with the claim that the same neurons were recorded across days and the fact that the monkey was not performing a learning task, the distribution of tuning differences of stable and orientation-tuned neurons across days was highly significantly different (Wilcoxon rank sum test, n1 = 79, n2 = 582, p < 10^-34) from the distribution of tuning differences across different neurons.
Our results show that using only waveform information it is possible to reliably track stable neurons across days with a limited type I error probability. This statistical approach is particularly important since, in a learning experiment, properties of neurons such as orientation tuning are potentially changed and therefore cannot be used to evaluate stability.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Recording chronically from the same neurons in awake, behaving primates150171542142727ASEckerPBerensGAKelirisNKLogothetisASToliasGöttingen, Germany2007-04-0012227th Meeting of the German Neuroscience Society, 31st Göttingen Neurobiology ConferenceRecent advances in both electrophysiological recording techniques and hardware capabilities have enabled
researchers to simultaneously record from a large number of neurons in different areas of the brain. This opens
the door for a wide range of complex analyses potentially leading to a better understanding of the principles
underlying neural network computations. At the same time, due to the increasing amount of data with
increasing complexity, significantly more emphasis has to be put on the data analysis task. Although
high-level scripting languages such as Matlab can speed up the development of analysis tools, in our
experience, a too large amount of time is still spent on (re)structuring and (re)organizing data for specific
analyses.
Therefore, our goal was to develop a system which enables experimental neuroscientists to spend less time on
organizing their data and more on data collection and creative analysis. We developed an object oriented
Matlab toolbox which supplies the user with basic data types and functions to organize and structure various
types of electrophysiological data. By using an object oriented, hierarchical layout, basic functionality, such as
integration of metadata, or storage and retrieval of data and results, is implemented independent of specific
data formats or experimental designs. This provides maximal flexibility and compatibility with future
experiments and new data formats. All data and experimental results are stored in a database, so the
experimenter can choose which data to keep in memory for faster access and which to save to disk to save
resources. Additionally, we have created an extensive library of basic analysis and visualization tools that can
be used to get an overview of the data.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de//fileadmin/user_upload/files/publications/EckerTolias_2007_ADataManagement_4272[0].pdfpublished-1222A Data Management System for Electrophysiological Data Analysis150171542142737PBerensGAKelirisASEckerNKLogothetisASToliasGöttingen, Germany2007-04-007357th Meeting of the German Neuroscience Society, 31st Göttingen Neurobiology ConferenceOscillations in the local field potential (LFP) are abundant across species and brain regions. The possible
relationship of these low-frequency extracelluar voltage fluctuations with the activity of the underlying local
population of neurons remains largely elusive. To study this relationship, we used an array of chronically
implanted tetrodes spanning a distance of 700 &#956;m and simultaneously recorded action potentials from multiple
well-isolated single units, multi unit activity (MUA) and LFP from area V1 of the awake, behaving macaque.
Moving and static gratings of different orientations were used for visual stimulation.
In agreement with previous studies we find that the increase of LFP gamma-band power is a function of the
orientation of the stimulus. However, the power of the gamma-band contains much less information about the
orientation of the stimulus than the MUA and SUA recorded at the same site (Figure 1A). The average
discriminability d&lsquo; between preferred and orthogonal orientation was 2.46 for MUA, 2.45 for SUA and 1.01
for the LFP. Moreover, in contrast to recent results from area MT (Liu and Newsome, 2006) we find only a
weak correlation between the preferred orientation of the MUA tuning function and that of the LFP (Figure
1B, different colors indicate different animals). Interestingly, all nearby LFP recording sites in our array were
tuned to a similar orientation while the preferred orientations of MUA tuning functions were widely scattered.
These results suggest that the power of LFP signals does not capture local population activity at the scale of
orientation columns in area V1.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de//fileadmin/user_upload/files/publications/T16-4C_[0].pdfpublished-735Orientation tuning of the local field potential and multi-unit activity in the primary visual cortex of the macaque150171542143607GAKelirisNKLogothetisAToliasSantorini, Greece2006-06-0058AREADNE 2006: Research in Encoding and Decoding of Neural EnsemblesUnder certain stimulus conditions we encounter pronounced perceptual suppression of
suprathreshold visual stimuli. The brain mechanisms underlying these phenomena are poorly
understood. Binocular rivalry (BR) and Binocular Flash Suppression (BFS) provide us excellent
behavioural tools to study this phenomenon. During these paradigms visual stimuli are completely
extinguished from our awareness for a substantial amount of time despite being physically
present on our retinas. Therefore, we can study the dissociation between the neural responses
that underlie a mere sensory representation of the visual input and what is perceived. Primary
visual cortex (V1) has been implicated as an important candidate for the site of perceptual
suppression. However, interestingly electrophysiological studies in V1 have found only a very
small percentage of neurons to be correlated with the percept[1]. In contrast, human fMRI
studies[2,3] have shown that the BOLD signal during such perceptual alternations modulates
almost as much as when the stimuli are non-ambiguously presented separately. These
contradicting results led to the speculation that the local field potential (LFP) signals, which have
been shown to correlate with the BOLD signal, will also show correlations with perception in
agreement with the BOLD results and thus potentially solve the apparent controversy. To this
end, a recent study[4] claimed that low frequency (<30Hz) LFP signals in V1 correlate well with the
subjective experience of macaques during BR.
We have used BFS and recorded neural activity from large populations of well-isolated single
neurons (SUA) from V1 using chronically implanted and non-chronic tetrodes in awake behaving
macaques. In addition to the SUA we also simultaneously recorded multi-unit (MUA) and LFP
signals. In agreement with previous electrophysiology experiments we find a very small
percentage of single neurons (12%, t-test: p<0.05) as well as MUA sites (15%) to be correlated
with the animals¹ percept during the binocular presentation of two gratings of orthogonal
orientations. Interestingly, an even smaller percentage (7%) of gamma-band LFP sites show a
significant modulation and no other LFP band (e.g. alfa or beta-bands) showed stronger
perceptually related modulation. In addition, the amplitude of the normalized population response
in all three signals shows a small fractional modulation in comparison with the monocular
presentation of the gratings (see figure). We therefore conclude that the activity in V1 is not a
good predictor of the perceptual alternations at least using the classical simple measures of firing
rate and power modulations of the signals.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-58Perceptual Suppression in area V1 of the Macaque150171542139497PBerensASEckerAHoenselaarGAKelirisAGSiapasNKLogothetisASToliasSantorini, Greece2006-06-0039AREADNE 2006: Research in Encoding and Decoding of Neural EnsemblesOscillations in the local field potential (LFP) are abundant across species and brain regions. The possible
role of these oscillations in information processing in the primary visual cortex (V1) of the macaque still
remains largely elusive despite that V1 is one of the most extensively studied brain areas. To this end, we
used chronically implanted, multiple tetrodes and recorded the spiking activity of single neurons and LFPs
from area V1 of the awake, behaving macaque. Moving and static gratings of different orientations were
used for visual stimulation.
In agreement with previous reports we find that the increase of the LFP gamma-band power is a function
of the orientation of the stimulus. Surprisingly though, there is only a weak correlation between the peak
of the multi-unit spiking activity orientation tuning functions and the peak of the orientation tuning function
of the gamma-band power of the LFP. There is however a different kind of relationship between spikes
and LFP. Namely, the timing of the spikes is not randomly distributed in time but instead is locked to the
phase of the gamma-band of the LFP. Specifically, the spikes of 60 out of 151 well-isolated single units
showed significant phase locking to the LFP (P<0.05, circular Rayleigh test). On average, the spikes occurred
on the downward slope of the LFP oscillation. In contrast to the presence of phase precession reported
in the rat hippocampus, the phase tuning in V1 is stable over time. Specifically, the preferred
phase of the spikes does not seem to change over time during the presentation of the stimulus. Moreover,
the preferred phase is not significantly modulated as a function of the orientation of the stimulus (Figure
A).
This temporal structuring of the spiking activity of neurons in V1 could allow coding of information in the
temporal regime (Panzeri & Schultz, 2001). In addition it could also potentially synchronize populations of
neurons (Fries 2005). We are currently investigating these conjectures.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-39Spikes are phase locked to the gamma-band of the local field potential oscillations in the primary visual cortex of the macaque1501715421ToliasEKSSL20067ASToliasAEckerGAKelirisTGSiapasSMSmirnakisNKLogothetisSalt Lake City, UT, USA2006-03-0013Computational and Systems Neuroscience Meeting (COSYNE 2006)Despite recent progress in systems neuroscience, basic properties of the neural code still remain obscure. For instance, the responses of single neurons are both highly variable and ambiguous (similar responses can be elicited by different types of stimuli). This variability/ambiguity has to be resolved by considering the joint pattern of firing of multiple single units responding simultaneously to a stimulus. Therefore, in order to understand the underlying principles of the neural code it is important to characterize the correlations between neurons and the impact that these correlations have on the amount of information that can be encoded by populations of neurons. Here we applied the technique of chronically implanted, multiple tetrodes to record simultaneously from a number of neurons in the primary visual cortex (V1) of the awake behaving macaque, and to measure the correlations in the trial-to-trial fluctuations of their firing rates under the same stimulation conditions (noise correlations). We find that, contrary to widespread belief, noise correlations in V1 are very small (around 0.01) and do not change systematically neither as a function of cortical distance (up to 600 um) nor as a function of the similarity in stimulus preference between the neurons (uniform correlation structure). Interestingly, a uniform correlation structure is predicted by theory to increase the achievable encoding accuracy of a neuronal population and may reflect a universal principle for population coding throughout the cortex.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-13Structure of interneuronal correlations in the primary visual cortex of the rhesus macaque150171542137227GAKelirisSMSmirnakisASToliasNKLogothetisWashington, DC, USA2005-11-0035th Annual Meeting of the Society for Neuroscience (Neuroscience 2005)Motion processing is a fundamental property of the visual system. Classical electrophysiology studies in the macaque as well as fMRI studies in the human have revealed an extensive network of visual areas that contain neurons selective for direction of motion. Recent evidence from macaque fMRI (Tolias et al., J Neurosci 2001) and electrophysiology (Tolias et al. Nat Neurosci 2005) suggests that the direction-of-motion selectivity of macaque visual areas is not a fixed property but can change dynamically as a function of the state of adaptation to a moving stimulus.
Here we used a visual motion adaptation paradigm to study the direction-of-motion selectivity of human visual areas with functional magnetic resonance imaging (fMRI). The visual stimuli we used consisted of expanding/contracting dot kinematograms at 100% coherence. These were presented passively while the subject performed an attentionally demanding task at the fovea. After moving unidirectionally (expanding or contracting) for about 160 sec the kinematogram abruptly reversed direction of motion. By measuring the blood oxygen level dependent (BOLD) signal response elicited by the direction of motion reversal and comparing it to the initial response elicited when the adapting stimulus turns on, we were able to assess the degree of direction-of-motion selectivity in the various visual areas.
We found that an extensive network of visual areas shows BOLD rebound when the direction of motion reverses after adaptation, including areas that according to classical electrophysiology do not show strong direction-of-motion selectivity (for example area V4). Our results agree qualitatively with the findings in the macaque (Tolias et al., J Neurosci 2001), and together underscore the dynamic nature of functional cortical architecture.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Directional selectivity of human visual areas after adaptation to motion stimuli: an fMRI study150171542133277GAKelirisSMSmirnakisNKLogothetisASToliasSan Diego, CA, USA2004-10-0034th Annual Meeting of the Society for Neuroscience (Neuroscience 2004)The ability to detect motion in our environment is a crucial function of the visual system. Motion processing is usually studied by comparing the activity elicited by various motion stimuli relative to a baseline (“no-movement”) condition. However, during natural vision the sensory input is not broken into a series of discrete presentations that are simply switched on and off. By using a motion adaptation paradigm we studied how stimulation history influences the directional selectivity of single neurons in area V4. We found that V4 neurons which classically would be thought as non-directionally selective can in fact acquire directional selectivity after adaptation. We recorded from area V4 of two monkeys using tetrodes and characterized the directional tuning properties of single units using drifting coherent random dot patterns. In agreement with previous studies we find that the majority of area V4 neurons are weakly tuned to the direction of motion when their properties are characterized using the classical stimulation paradigm. The same neurons though, express stronger directional tuning if previously adapted to a moving stimulus for a period of one second. To quantify the amount of directional information present in the activity of V4 neurons we used a Bayesian population decoding method to predict the direction of motion of the stimulus trial by trial using the activity of a population of neurons in a four hundred millisecond window. The average test error dropped significantly when computed after adaptation. It is important to characterize the properties of neuronal circuits under adaptation to better understand the mechanisms of natural vision.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Motion processing in area V4 revealed with adaptation: Tetrode recordings in the awake, behaving macaque1501715421MoutoussisKKL20047KMoutoussisGKelirisZKourtziNKLogothetisSan Diego, CA, USA2004-10-0034th Annual Meeting of the Society for Neuroscience (Neuroscience 2004)The relationship between brain activity and conscious visual experience is central to our understanding of the neural mechanisms underlying perception. Binocular rivalry, where monocular stimuli compete for perceptual dominance, has been previously used to dissociate the constant stimulus from the varying percept. We report here fMRI results from humans experiencing binocular rivalry under a dichoptic stimulation paradigm that consisted of two drifting random dot patterns with different motion coherence. Each pattern had also a different color, which both enhanced rivalry and was used for reporting which of the two patterns was visible at each time. As the perception of the subjects alternated between coherent motion and motion noise, we examined the effect that these alternations had on the strength of the MR signal throughout the brain. Across the different visual areas, we have found varying degrees of correlation between the neural activity and the visual percept. Areas V3A, V5 (MT) and LOC showed a much stronger activation when subjects perceived coherent motion than when they perceived motion noise. A similar but not as strong an effect was observed in area V3, whereas a much less pronounced difference between the two conditions was found in areas V1, V2 and V4. These results demonstrate that motion perception is able to modulate the activity of most visual areas known to be involved in motion processing. Instead of a clear distinction between ‘processing’ and ‘perceptual’ areas, we found a gradual increase in the correlation between neural and perceptual events as one moves towards the higher areas of the motion pathway. We thus conclude that the areas involved in the processing of a specific visual attribute are also part of the neuronal network that is collectively responsible for its perceptual representation.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0The involvement of different areas of the human visual brain in motion perception1501715421150171542233287KMoutoussisGAKelirisZKourtziNKLogothetisBudapest, Hungary2004-06-00e1023e102410th Annual Meeting of the Organization for Human Brain Mapping (HBM 2004)Introduction
One of the most fascinating problems in visual neuroscience is finding a direct relationship between brain activity
and perception. When a visual stimulus is presented to the eyes, it elicits a series of responses in many and
different parts of the visual system, from the retina to the ’higher’ cortical areas, leading to a conscious visual
percept. Dissociating which part of the visual brain activity is reflecting our perception is thus hard, since at the
same time this activity is directly related to the processing of the visual stimulus itself. To try and answer this
question, binocular rivalry has been used in the past, where the stimulus (which is different for each eye) remains
constant but the perception alternates between the two rivalring monocular inputs. In this way one can dissociate
the stimulus from the percept and, by studying the alternations in brain activation under such conditions, get an
insight into which brain areas correlate their activity with what the subject actually perceives.
Methods
Binocular rivalry was used in fMRI experiments that were performed on a Siemens Trio 3T system. A different
random dot kinematogram was shown to each eye, one consisting of red and the other of green dots. In one of the
kinematograms 50% of the dots moved in the same direction producing a coherent motion signal whereas in the
other all dots moved in random directions thus producing pure motion noise. As binocular rivalry developed
between red dots in one eye and green dots in the other, subjects inside the scanner used two different buttons to
report whether they perceived one the another color. In this way we could relate the BOLD signal we recorded in
the magnet to the subjects’ percept and investigate how motion perception is reflected in the cortical activation of
the various visual areas.
Results
Averaging the event-related time-courses across all subjects showed a range of different responses, with no
significant effect in areas V1, V2 and V4, only a slight difference in area V3, and a much more clear difference in
areas V3a, V5 and LOC (Fig. 1).
Discussion
In this study we were able to show that a number of visual areas are involved in motion perception. In general, the
more involved an area is in motion processing, the more it is modulated by motion perception, supporting the idea
that processing and perceptual areas are not distinct and separable, but rather the same areas are involved in both
processing and perception.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Neural correlates of motion perception in the human visual brain150171542127307GAKelirisSMSmirnakisZKourtziASToliasNKLogothetisOrlando, FL, USA2002-11-0032nd Annual Meeting of the Society for Neuroscience (Neuroscience 2002)Perceptual filling-in refers to the fading of stabilized retinal patterns and their replacement by non-stabilized surrounding patterns. We used functional magnetic resonance imaging (fMRI) to investigate neuronal correlates of perceptual filling-in induced by a dynamic random dot pattern. The stimulus consisted of a moving random dot pattern on dark background surrounding a region devoid of dots (artificial scotoma). The subjects fixated at an eccentrically located spot, and they reported the time of onset of filling-in by button press. We controlled for attention by dimming the fixation spot at random points in time, which the subjects reported via a separate button press. Catch trials in which the stimulus physically filled the artificial scotoma were interspersed with filling-in trials to gauge the subjects performance. General linear model techniques with appropriate predictors were used to define areas of interest for analysis. Filling-in trials for each subject were divided in two groups of 30 trial
s each, based on whether filling in occurred earlier (&amp;lt;8 s) or later (8-24 s) in a trial. The stimulus was identical for all trials. Preliminary results suggest that the fMRI signal from area V1 rises initially in both groups but then dips and remains low for the group with early filling-in. This suggests that filling-in is associated with a relative suppression of cortical activity. Other interpretations will be discussed.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0FMRI Correlates of Perceptual Filling-in in a Moving Random Dot Paradigm1501715421PapanikolaouKLPSLS201410APapanikolaouGAKelirisSLeeTDPapageorgiouUSchieferNKLogothetisSMSmirnakisKeliris2013_210GKelirisPapanikolaouKPSKPSLS201310APapanikolaouGAKelirisGTPapageorgiouYShaoEKrappEPapageorgiouUSchieferNKLogothetisSMSmirnakisKeliris201310GKelirisLeeKPSL201210SLeeGAKelirisAPapanikolaouSMSmirnakisNKLogothetis529010ABartelsTPanagiotaropoulosGAKelirisNKLogothetisToliasEKPPL200710ASToliasAEckerGAKelirisFPanagiotaropolulosSPanzeriNKLogothetis372310ASToliasGAKelirisASEckerAGSiapasSMSmirnakisNKLogothetis